• People's brains start to shrink in their 30s and 40s. 
• As they get even older, it becomes harder for them to process information and remember things.
• But people, and presidents, age differently because of their genetics and unique circumstances.
• Read more from Insider's "Red, White, and Gray" series.

Trent Lott, a Mississippi Republican, could tell the president of the United States couldn't place him.

It was 1988, and he'd approached President Ronald Reagan in the Cabinet room at the White House. The president had made it a habit to meet almost weekly with Lott and other lawmakers whenever Congress was in session. 

"I never will forget the look in his eyes," Lott said. "He looked at me almost like: 'I don't understand what you're saying. Who are you?'" 

Lott said such an exchange happened only once. Reagan, who would have been about 77 at the time, could have been having an episode of forgetfulness that's normal for people with advanced age, scientists say. Maybe he was just short on sleep. 

Or maybe Lott, who said Reagan was his favorite president to work with, was observing a change in cognitive function that only people close to the president could have noticed.

It's hard to know for sure, experts on aging told Insider. The public is generally in the dark when it comes to presidential health. US presidents aren't required to disclose such details, and any medical records they do make public are ones they've approved voluntarily.

Without making objective, detailed assessments from doctors public, presidents have opened themselves up to politicized speculation. People scoffed at the glowing hourlong media conference that President Donald Trump's White House doctor gave about his health. President Joe Biden hasn't once subjected his longtime doctor to questions from the press, breaking with decades of precedent.

What's left is rumors and what both laypeople and experts observe publicly.

When Lott approached Reagan in 1988, he was the minority House whip, someone responsible for rounding up votes who was therefore regularly in touch with the world's most powerful man.

He said the interaction stuck with him as "the first indication that I got that he was beginning to lose it."

Reagan publicly announced he had Alzheimer's disease five years after he left the White House. Anecdotes aside, historians haven't found evidence Reagan had dementia during his presidency that would have affected his ability to make decisions. Moreover, diseases causing dementia can take many years to damage the brain enough for symptoms to start showing regularly.

The Reagan years introduced the nation to the idea that a president didn't have to be middle-aged. In 2017, Trump surpassed Reagan as the oldest president at the time elected, at age 70.

Then, last year, Biden finally realized his lifelong quest to become president, at 78 years old. Reagan was 77 when he left the White House.

Meanwhile, the three top Democrats in the House — Speaker Nancy Pelosi, Majority Leader Steny Hoyer, and Majority Whip James Clyburn — are all in their early 80s. Senate Minority Leader Mitch McConnell is 80. Senate Majority Leader Chuck Schumer is 71. And the average age of a member of Congress is 61.

Four months in the making, Insider's "Red, White, and Gray" series explores the costs, benefits, and dangers of life in a democracy helmed by those of advanced age, where issues of profound importance to the nation's youth and future — technology, civil rights, energy, the environment — are largely in the hands of those whose primes have passed.

The nearly 330 million Americans affected by these leaders' decisions cannot, of course, get inside their heads. But science on how the brain changes over time has a story to tell.

Our ever-shrinking brains

The outward signs of aging are easy to see. 

You get wrinkles and gray hair. The older you get, the more likely you are to develop conditions such as hearing loss and arthritis. Injuries can take longer to heal.

Then there are the changes inside the brain, the most complex organ in the human body. They start to happen earlier in life than many people might expect. 

People start to have trouble remembering words for things when they're in their 30s, S. Jay Olshansky, a leading expert on aging at the University of Illinois Chicago, told Insider. It's a familiar experience for many people, having a word on the tips of their tongues while trying to jog their memories.

There's a biological explanation: During our 30s and 40s, our brains have already begun to shrink.

The parts of the brain responsible for cognitive functions shrink more than other parts. And the wrinkled outer part of the brain thins. These changes generally increase in speed when people are in their 60s.

As you get older, it subtly and gradually becomes harder to recall where you parked your car or put your phone. Retaining information after people tell you their name or give you their phone number becomes more difficult.

Recalling facts or things that happened to you becomes a greater challenge. You don't sleep as well or as deeply. Multitasking and paying attention for long periods of time taxes your energy.

"We tend to process things slower," Dr. G. Michael Harper, the president of the American Geriatrics Society, said. "It takes more time to learn new things."

These are changes that might, for example, affect lawmakers' performance in Congress. 

They regularly fly or drive from their districts to Washington, DC, and give floor speeches, meet with constituents and lobbyists, and conduct hearings. Votes can sometimes run until 2 a.m., and House hearings can last for hours.

And fundraising is an intrinsic part of the job — whether it's at a crowded gala in Palm Beach, Florida, or during back-to-back phone calls to donors at the Democratic National Committee office just a few blocks from the US Capitol.

Members of Congress do have the option of leaning on their staff, but voters didn't elect legislative aides to represent their interests in Washington.

Redefining 'normal'

Among the most obvious differences between an average person in their 50s and someone in their 80s can be seen during a memory test, Emily Rogalski, an associate director of the Mesulam Center for Cognitive Neurology and Alzheimer Disease at Northwestern University, said. 

Doctors read a list of 15 unrelated words to patients. About 30 minutes later, they'll ask patients to repeat the words.

It's perfectly normal for someone in their 50s or 60s to remember nine of those words, Rogalski said. And it's considered normal for someone in their 80s to remember only five of them.

"What we call average or normal adjusts with age," she said. 

Having trouble with name recall is typical in aging and has "nothing to do with cognitive function," Dr. Nir Barzilai, the director of the Institute for Aging Research at the Albert Einstein College of Medicine, said.

"You have to separate those," he said. "Slowness is also part of aging, but it's not part of cognitive function."

But normal cognitive decline can progress into what's known as mild cognitive impairment, a condition that can stay the same, improve, or lead to dementia. People can forget important events, lose their train of thought, or become more impulsive. They might feel overwhelmed by making decisions — and their friends and family tend to notice the changes.

The risk for dementia starts to increase after 65, then doubles every five years, according to the Centers for Disease Control and Prevention. By 85 and up, the chances are about one in two that you'll have either mild cognitive impairment or dementia, Harper said.

With Alzheimer's disease, the most common type of dementia, people have a harder time functioning on their own as abnormal proteins build up in their brain. Scientists think this buildup causes plaques and that a second protein forms tangles. Together, they block communication between nerve cells, the thinking goes.

With Alzheimer's, people can repeat questions, get lost in familiar places, and forget how to use everyday items. They'll eventually be unable to carry out everyday tasks, from dressing to bathing, and may not recognize their loved ones.

Should a president become incapacitated with such a condition or otherwise become unfit to serve, the Constitution's 25th Amendment — ratified in 1967 — details steps for transferring power to the vice president.

Some Trump detractors called on Congress and Trump's Cabinet to remove him via the 25th Amendment, and the US House in January 2021 formally requested Vice President Mike Pence do so. (He declined.) 

The House's January 6 select committee has questioned Trump cabinet members on how seriously they considered this option. 

For Congress, Article 1, Section 5, of the Constitution empowers lawmakers to craft rules for themselves, including procedures for expelling members for "disorderly behavior." Since the nation's founding, the Senate has expelled 15 of its members and the House five — none of them were removed for reasons expressly related to mental decline or incapacitation.

But some members have voluntarily stepped down for health reasons.

Sen. Johnny Isakson, a Republican of Georgia, had Parkinson's disease and resigned at the end of 2019 at age 74. When Sen. Orrin Hatch of Utah, the longest-serving GOP senator in history, retired at 83, he said: "Every good fighter knows when to hang up the gloves."

More recently, congressional colleagues of Democratic Sen. Dianne Feinstein of California anonymously told the San Francisco Chronicle that they're concerned about her, saying her memory was deteriorating. Feinstein has not announced any plans to retire — she's up for reelection in 2024 — and said she faced a painful and difficult year as her late husband was dying of cancer.

A 1973 closeups of Joe Biden, when he was a Democratic US senator from Delaware. Getty Images

Aging has positives for the brain 

For people in public service, staying in the game can give them tremendous purpose at any age. While some people might thrive during retirement, others appear to deteriorate overnight, experts on aging say.

Social isolation and loneliness can lead to negative effects, Rogalski said, and the brain likes to learn things and be exposed to challenges.

The organ also improves in some ways as you get older. You become better at managing conflicts and letting disagreements roll off your back that might have upset you when you were younger.

Attribute this to how your brain is processing your experiences. What's known as "crystallized intelligence" grows. The term refers to the knowledge that comes after you've had various experiences, which can lead to wisdom.

"If you think about old politicians, they may have more difficulty with problem-solving, but they'll have a lot more stored knowledge and past experience they can rely on," Harper said.

Lott said he recognized this during his 34 years in Congress.

"When you're young and impetuous, you might be more inclined to charge into the valley of a major fight," he said. "As you grow older and get smarter, you might say, 'Wait a minute. Let's see if we can't work out something here.' You find a compromise that's acceptable. I do think there is a real need for youth and energy but also even more need for experience and wisdom."

"The key is to meld the two," he added.

In Congress, marking up bills requires going back and forth to arrive at a compromise. The more members know about their colleagues and how events play out, the better they can be at legislating.

Republican Gov. Spencer Cox of Utah, who was first elected to office as the mayor of Fairview at 29, told Insider he noticed changes in himself now that he's 47.

As a young politician, he said he was "a little brash at times." He said he couldn't understand why people didn't see things the way he did. That can cause both policymaking and governing to suffer, he added.

"I was pretty quick to attack people early on, people who didn't see things the way that I did," he said. "I rarely gave people the benefit of the doubt. And that's something that, over time, I've learned from. I've learned to be much more patient and to think two or three times before I tweet or say something."

Another politician who reflected deeply on aging's positives was Democratic Sen. Robert Byrd of West Virginia, a forerunner of the much-talked-about Democratic Sen. Joe Manchin.

In 2007, Byrd addressed questions about his age — raised in a Times West Virginian news story — during an impassioned 14-minute floor speech.

Byrd, then 89, with his hands visibly shaking as he turned the pages of his speech, acknowledged his white hair, wrinkled face, and need for a cane. But getting older, he said, was "a state worthy of respect."

"Our brains are still sharp, but our tongues are slower," he said. "We have learned, sometimes the hard way, to think before we speak. I hope whatever we have to say is worth the wait. Many good things are worth the wait."

Aging is relative 

The changes in the brain laid out in this story occur as described for millions of Americans. 

But there are exceptions. Not all 80-year-olds are the same. While some use wheelchairs, others run marathons, said Steven Austad, the senior scientific director at the American Federation for Aging Research.

If you try to interview the 81-year-old Sen. Bernie Sanders of Vermont, for instance, you better be wearing comfortable shoes so you can keep up with him because he walks everywhere with urgency. You'll have to raise your voice to speak with Sen. Chuck Grassley of Iowa, who's 88, but don't challenge him to a push-up contest unless you want to embarrass yourself.

Experts on aging become frustrated when people take their experiences of aging — or that of their parents or grandparents — and project them onto others, including presidents and members of Congress.

They say that just because you can't imagine running the country at 80 doesn't mean someone else can't. Some people thrive on the challenges of the job, even as they get older.

"I personally can't imagine the stress of that kind of political office, but that's just not my personality," Austad said. "Clearly, there are people who have been in politics all their lives who don't seem to think of it as stressful."

John Rowe, a professor of health policy and aging at Columbia University's Mailman School of Public Health, has conducted studies finding that for some people over 75, cognitive function can improve with age.

His research homes in on how the environments people grew up in, their lifestyles, and their genes factor into how they age.

Some people retain and even expand their creative powers — think John Williams composing movie scores at 90, the naturalist David Attenborough crisscrossing the world with new television shows at 96, or Nola Ochs earning a master's degree at 98. Queen Elizabeth II died on September 8 at age 96, and she remained decidedly visible and active, even in the hours before her death, when she appointed Liz Truss as the United Kingdom's new prime minister.

Health disparities are the biggest problem with aging in America, Rowe said.

People who go to college are less likely to smoke and more likely to exercise, work, and have healthcare coverage than people who don't. On average, white Americans live longer than Black Americans. 

In the US, Asian Americans have the longest life expectancy at birth, at 83.5 years, while American Indian and Alaska Natives have the shortest, at 65.2 years, CDC data from 2021 show. 

Stress, including from poverty, contributes to a more difficult aging process, Rowe said. Anxiety and depression also seem to be associated with the onset of cognitive impairment. 

"In general, if you look at the privileged, educated, well-off white populations, you know that they have a very different experience in aging than the group at the other end of the spectrum," Rowe said. "That's the group our elected officials are coming from."

But genetics can't be ignored. If your parents live a long time without certain impairments, chances are that you will, too.

Your genetic makeup can also override negative health habits. Barzilai from the Albert Einstein College of Medicine, who studies centenarians, once met a woman at her New York apartment when she was 100. All her siblings lived to be over the age of 102.

When he arrived at her apartment, she opened the door, cigarette in hand.

"Nobody told you to stop smoking?" he asked her.

She shrugged and replied, "All four doctors who told me to stop smoking died.'"

She would live to become 110.

So to Barzilai, the idea that Trump might still have a long life ahead of him — despite reports of his fast-food eating habits and a body-mass index once categorized as obese — isn't far-fetched.

In this Friday, January 12, 2018, file photo, President Donald Trump shakes hands with then-White House physician Dr. Ronny Jackson as he boards Marine One to leave Walter Reed National Military Medical Center in Bethesda, Maryland, after his first medical check-up as president. Jackson is now a member of Congress. Carolyn Kaster, File/AP Photo

Testing the president's brain 

People who make it to 80 with the same kind of cognitive abilities they had when they were middle-aged are called "superagers." An estimated 15% of people fall into this exceptional category.

Their performance on the memory test where doctors ask patients to recall words will be different from that of the typical 80-year-old. They'll be able to recall the same number of words as people in their 50s and 60s, Rogalski of the Mesulam Center said.

So could Trump and Biden be part of this exclusive club? 

Olshansky, who has studied superagers but doesn't have access to all the medical information from both men, said he "can't judge" but that there was "no reason why someone can't be president in their 80s."

"Both seem to have cognitive functioning and probably qualify," he said of Biden and Trump.

If the two men have another matchup in 2024 — something that's entirely plausible — age will be an inescapable theme.

Conservatives routinely suggest Biden's mental faculties are declining and that he's, in the words of the Fox News host Tucker Carlson, "cognitively unable to serve." Some voters have concerns, too, according to a Politico-Morning Consult poll. 

Liberals and "never Trumpers" have lobbed all manner of descriptors at Trump: "insane," "psychopath," "lunatic," "deranged."

Francis Shen, who teaches psychiatry at Harvard Medical School's Center for Bioethics, would like to see information about political leaders' cognitive health made public. 

He said it could be similar to how members of Congress have to disclose their personal financial investments.

Cognitive health should be no different, he said, because it also might affect the way presidents and members of Congress make important decisions. 

"We have many politicians and judges who continue to serve well into later life," Shen said. "And we have no system for objectively assessing their cognitive health. And, to me, that's a problem."

Even if given that information, he said, voters may still decide to cast their ballots for particular politicians because they like them.

"It shouldn't be disqualifying. But it should be part of the information we know" when people go to vote, he said, adding: "Especially since it is knowable. This does not have to be a mystery."

The White House and Trump's post-presidential office did not respond to requests for comment. Trump hasn't said whether he's running again, but in a phone call with the New York Post gossip columnist Cindy Adams, he said, "just remember what I'm telling you ... 78 is not old."

Seventy-eight would be Trump's age at the start of a second term.

The prospect of a second Trump term is one of the main factors driving Biden toward running again, The Washington Post reported. But in a poll from The New York Times and Siena College, 64% of Democratic respondents said they didn't want Biden to run for the White House — and age was the top concern.

Asked about it in July, Biden singled out the portion of the poll that said 92% of Democrats would vote for him again if Trump was his opponent in 2024.

"They want me to run," Biden​ said. "Read the poll."​

Warren Rojas and Darren Samuelsohn contributed reporting. 

March 28, 2023: A previous version of this story incorrectly said that white Americans had greater longevity than Hispanic Americans.

Watch: Watch how Joe Biden has aged from 1974 to 2022

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