AMNA NAWAZ: As America braces for a rematch between President Joe Biden and former President Donald Trump, the ages of both men are troubling voters in this election year.

Laura Barron-Lopez looks at the questions many have around aging, memory and the presidency.

LAURA BARRON-LOPEZ: Amna, age is much more than just a number for both President Biden and former President Trump.

It's a potential political liability.

We're going to focus on some of the questions people have.

But, first, we're going to hear from some older Americans, voters who are 70 and above, to get their perspective on this moment and what amounts to an ongoing national conversation about age.

DANIEL CABRERA, California: My name is Dan Cabrera.

I live in Southern California, and I am 71 years old.

And neither candidate really has got it together, in my view.

They both have serious shortcomings when it comes to their clarity, their intellect.

DENNIS TAYLOR, Montana: So I'm Dennis Taylor.

I live in Helena, Montana, and I'm 77 years old.

The whole thing that we're talking about in this election is a number between, what, 77 -- that's my age, the same as Donald Trump -- and 81.

When Chuck Grassley was running for reelection at 90 for another six-year term, which his voters overwhelmingly gave him, nobody said a word.

SUSAN WARD, North Carolina: I'm Susan Ward.

I live in Asheville, North Carolina, and I'm 70 years old.

I'm an active 70-year-old, and I wouldn't have the stamina to be president of the United States.

And so I do think it's worth thinking about.

At the same time, this is what we have got.

MARY ALICE SHAKER, Virginia: My name is Mary Alice Shaker.

I live in Virginia, Richmond, Virginia, and I'm 77 years old.

I think President Biden is very competent and very, very knowledgeable.

And if he gaffes a little bit once in a while, I don't think that affects his ability.

For the next four years, I'm not so sure.

DENNIS TAYLOR: Nobody said anything about Warren Buffett or Cicely Tyson or Jane Fonda or Dr. Fauci or Mick Jagger or Harrison Ford.

I mean, think of all the people who in their 80s and 90s contributed to the society.

Can you imagine saying that Martin Scorsese needs to take a test before he can do this next movie?

DANIEL CABRERA: You can look at other major world leaders, in my view.

And Zelenskyy is a stellar example, Macron, Rishi from the U.K., et cetera.

These guys are younger, more energetic, more able, in my view, than either of the two candidates we currently have as leading Republican and Democratic presidential candidates.

SUSAN WARD: I have seen some people list that medical cognitive test kind of requirement.

And, on one hand I could say, oh, that's probably a good idea, because then we'd have some more information about older people running for office.

But, on the other hand, I think to myself, do we do that just for presidents?

DENNIS TAYLOR: We worry about implicit bias, where we may not know that we have certain biases against certain kinds of people or stereotypes or prejudice that we may have or harbor.

In the case of ageism, it's like an explicit bias, and everybody's cool with it.

MARY ALICE SHAKER: I watched with interest the Ruth Bader Ginsburg and the Dianne Feinstein, and if they had only retired when they were supposed to retire, a lot of the mess we're in right now wouldn't be there.

You have to know when it's time.

If I were in their shoes, I would step aside.

I would step aside and let younger people take over.

LAURA BARRON-LOPEZ: Following verbal stumbles on the campaign trail, polls have consistently shown voters are concerned about the mental fitness of both leading presidential contenders.

In a recent poll, an NBC News survey found that 76 percent of voters had major concerns about President Biden's physical and mental health.

Meanwhile, 48 percent of voters had the same major concerns about former President Trump.

To help us understand more about what happens to our brains as we age and for some perspective on these lapses from both men, I'm joined by two experts in memory and cognition.

Dr. Dan Blazer is a professor emeritus of psychiatry and behavioral studies at Duke University.

Charan Ranganath is a professor of psychology and neuroscience at the University of California, Davis.

He's also the author of a new book releasing next week called "Why We Remember: Unlocking Memory's Power to Hold on to What Matters."

Thank you both so much for joining us.

Professor Ranganath, I want to start with you.

As we know, President Biden is 81 years old.

Former President Donald Trump is 77.

Help us sort through what we know about what happens to cognition, brain function, and memory as we age, particularly once we get over the age of 75.

CHARAN RANGANATH, University of California, Davis: Yes, there's actually -- it's fascinating because the work has really been evolving.

But the basic story is, on average, memory goes down with age.

I don't think that's a surprise for many viewers.

But if you actually look at different individuals, it really varies.

Some people, if you track them over time, they can go into their old age and they're super agers and they're fine, and other people do have a decline over time.

And so you really need to ask yourself what's going on with an individual person.

LAURA BARRON-LOPEZ: Last week, special counsel Robert Hur concluded no charges would be brought against President Biden for his handling of classified documents.

But the special counsel commented on the president's memory, saying that the president couldn't remember even within several years when his son Beau Biden died.

And then President Biden offered a stinging rebuke in response to Robert Hur.

But when he took a question from a reporter about Gaza, he mixed up the country of a foreign leader.

JOE BIDEN, President of the United States: As you know, initially, the President of Mexico El-Sisi, did not want to open up the gate to allow humanitarian material to get in.

I talked to him.

I convinced him to open the gate.

I talked to Bibi to open the gate on the Israeli side.

I've been pushing really hard -- really hard to get humanitarian assistance into Gaza.

LAURA BARRON-LOPEZ: President Biden clearly meant to say the president of Egypt, not of Mexico.

But, Dr. Blazer, do slips of memory like that signal any real deficits or an ability for a person to make hard decisions?

DR. DAN BLAZER, Duke University: I think that's the critical question.

First off, these types of slips are not uncommon with older persons, especially when they're having to recall a number of facts in a fairly brief period of time, and also when they're under the pressure of having to respond quickly and briefly to questions that are being posed.

That doesn't necessarily at all relate to their ability in judgment.

What -- another element of that judgment is something we call executive function.

That's ability to make judgments, to really see the big picture.

So, I think the bottom line for me is that you have to test these individuals in terms of how well they're doing their job, not how well they may perform in a particular situation.

If they slip on a particular topic, forget something, I think you have to look at the big picture.

LAURA BARRON-LOPEZ: Dr. Ranganath, if you watch President Biden now versus eight years ago, there's no doubt that his speech is at times a little less crisp.

He might take some longer pauses.

But what can we know and what don't we know by watching him?

CHARAN RANGANATH: So first of all, I just want to be clear that I'm a scientist, and so I can't diagnose anyone.

What I can say is that these verbal slips that Dr. Blazer brought up, and you said it perfectly, they're not even memory slips, per se.

They're really difficulties, and you just get a little bit slower to come up with words, and, sometimes, you're slower to catch the errors when they happen.

So I wouldn't even call those memory slips, per se.

I think, sometimes, people judge the appearance of something like that, and they think that there's some memory problem, but that's not really a memory problem.

I know President Biden had a stutter when he was growing up, and that actually also demands more executive function just to articulate.

So I think that might be factoring in too.

But, again, I totally concur with Dr. Blazer that you really need to ask these questions in a real way, as opposed to just superficial observations.

LAURA BARRON-LOPEZ: And, Dr. Blazer, Trump himself has repeatedly mixed up people as well, including Nikki Haley and Nancy Pelosi, when he was talking about the January 6 insurrection.

DONALD TRUMP, Former President of the United States (R) and Current U.S. Presidential Candidate: By the way, they never report the crowd on January 6.

Nikki Haley, Nikki Haley, Nikki Haley, Nikki Haley -- you know, they -- do you know they destroyed all of the information, all of the evidence, everything, deleted and destroyed all of it, all of it, because of lots of things.

Like, Nikki Haley is in charge of security.

We offered her 10,000 people, soldiers, National Guard, whatever they want.

They turned it down.

LAURA BARRON-LOPEZ: Dr. Blazer, when you watch these two men, Mr. Trump and Mr. Biden, is it possible at all to gauge their fitness for office?

DR. DAN BLAZER: I think it's difficult.

I mean, you might look in terms of sort of the tone of what they're saying, people will make judgment based on that, but in terms of their cognitive function, I think it's quite difficult.

First off, I think, giving one test or two tests, it's just going to be totally insufficient to do that.

You need tests that are serial over time to really determine if there really has been significant deterioration.

But, in addition to that, you need a battery of tests.

And, personally, I think that what is most helpful is an honest opinion from the people surrounding them who might give the type of information that would say, we believe this person is actually functioning well or not.

In a political environment, that's going to be very difficult to obtain.

LAURA BARRON-LOPEZ: Dr. Ranganath, generally speaking, what are typical signs of cognitive decline, and when does it tend to impact people and how?

CHARAN RANGANATH: One of the interesting things is, I have seen many, many patients with memory disorders, and some of them are even on the younger side, and they can be very articulate, and yet they have disabling memory disorders.

So, the surface looks can be deceiving.

But I like to say there's forgetting and there's forgetting.

So just in terms of lowercase forgetting, that would be the day-to-day stuff that I think we all do, where you have something, and you know it's there, but you just can't find it, and then maybe even a few days later, that memory pops up into your head.

And so in a technical term, we call that retrieval failure.

And so these retrieval failures are benign, but they happen a lot as you get older.

Now, separate from that is what I would call forgetting with a capital F. And when you -- that -- by that, what I mean is that basically the memory is not there.

Maybe it was never formed or maybe it's just gone.

So, for instance, if you misstated, for instance, Egypt and Mexico, or if you had trouble remembering the year that something happened, I would say, well, that's more in the benign category.

But if you forget that you met the president of Mexico or if you forget significant events from these important times in your life, then I would say that's a real memory disorder, because those are things that I would expect people to remember.

LAURA BARRON-LOPEZ: Dr. Blazer, you mentioned the political environment.

And there's been a lot of words thrown around this election cycle loaded with partisanship, words like senile, deranged, dementia.

Do you think any of these have validity?

DR. DAN BLAZER: Well, they might have validity in some context, but I'm not certain they have validity here.

That's -- I think that's going to be one of the major challenges we see over the next year.

And that is, people are going to throw terms around that neuroscientists, that physicians are going to be -- they have used traditionally for quite different reasons than what are being used in this particular situation of two candidates who are in the public eye and who are being asked to make statements about situations that are complex.

And they have to make those statements on the spot.

And they're not given a chance to really correct areas that they may make.

LAURA BARRON-LOPEZ: Professor Ranganath, is there a word of advice that you would give to the public or to the press about making assumptions in this moment about memory or age or verbal lapses?

CHARAN RANGANATH: Absolutely.

I think, when people hear something like a loaded term like an elderly man with a poor memory, it just activates all these stereotypes that people have about aging is this inevitable slide towards senescence, right?

There's actually a lot of abilities that remain stable or even get better with age.

So, for instance, knowledge, like the kind of knowledge that you would hope a president would have, that remains stable or can even improve.

Likewise, you see things like compassion and emotion regulation that can be improving with age, or at least remain stable.

I think a lot of what people judge is based on surface characteristics that are dominated by confidence or by physical presence, rather than the more substantive issues. '

LAURA BARRON-LOPEZ: Dan Blazer of Duke University and Charan Ranganath of U.C.

Davis, thank you so much for your time.

DR. DAN BLAZER: You're welcome.

CHARAN RANGANATH: Thanks for having me.