In the wake of last week’s presidential debate between the 78- and 81-year-old candidates—and the impression among some that President Joe Biden looked “old and frail,” with at least one public call for cognitive testing—much of America has had age on the brain.

But what does age actually do to the brain? Fortune consulted with experts on aging to get a clearer picture.

The incredible shrinking cortex

“The brain undergoes many changes associated with aging, and one of them is the shrinkage of what we call the outer layer of the brain, or the cortex,” Emily Rogalski, professor of neurology at the University of Chicago and director of its Healthy Aging & Alzheimer’s Research Care Center, tells Fortune.

The cortex, she explains, is like the bark on a tree, and is the layer where brain cells live.

“It’s really important to our thinking and our communication,” she says, and its shrinking tends to occur in areas related to memory, and tends to be correlated with changes in memory—which is at its peak performance, believe it or not, when we are just in our 20s or early 30s.

Also vulnerable as a result are skills of attention and executive functioning. “And all of these things are interrelated in a way, because you need to have good attention in order to remember something,” Rogalski says. “Our cognitive functions don’t just sit on little islands of, here’s memory and here’s attention, and there’s no interaction. It’s a complex system.”

Age-related memory loss is normal

A recent McKnight Brain Research Foundation survey, points out Patricia Boyle, professor of psychiatry and behavioral sciences at Rush University and a neuropsychologist with the Rush Alzheimer’s Disease Center, found that 87% of Americans are concerned about experiencing age-related memory loss and a decline in brain function as they grow older.

“But, what many don’t know is that age-related memory loss is not always a sign of a serious cognitive problem,” Boyle tells Fortune. “Most people do not understand that age-related memory loss is usually associated with mild forgetfulness and is a normal part of brain aging and not necessarily a sign of a serious memory problem.”

Some signs of normal aging, she says, include:

• Making a bad decision occasionally

• Missing a monthly payment

• Losing track of time

• Not being able to find the right words

• Losing things around the house

“As we get older, it is normal to see signs of cognitive aging just like it’s normal to see the physical signs of your body aging, like moving slower or more aches and pains,” Boyle says.

Brain shrinkage does accelerate when you’re older

Brain volume continues to decrease as we age—including the frontal lobe and hippocampus, the areas responsible for cognitive functions—with the rate of shrinkage increasing by around age 60.

“With aging, we increase our risk for many diseases just by getting older,” which makes sense, Rogalski explains, if you think about wear and tear and the increasing vulnerabilities of our body—and the fact that, unlike with hips or knees, there are no brain replacements.

Aging brings the possibility of one of two types of atypical loss of cognitive function, notes Dr. John Rowe, a Columbia University Mailman School of Public Health professor of health policy and aging: dementia and mild cognitive impairment (MCI), “an age-related change that occurs in between 12% and 18% of older people, over 65,” he says. “And what is reflected in day-to-day living is that people become more forgetful, they lose things, they miss appointments, and this can have an impact on your day-to-day function.” MCI, he adds, progresses to dementia in about 10% of people per year.

Some older adults are performing at high levels

Rogalski stresses that an important part of looking at aging is to not just dwell on the things that go wrong, but new opportunities. “A huge challenge with aging is actually the stigma associated with aging and the expectations that we put on individuals as they age—that there is no trajectory but down—and that we take away activities and responsibilities that people can do.”

And that’s a problem in some new, luxury assisted-living facilities, she says, which provide services from room service to laundry folding. “It turns out that many of these daily activities that we do, such as washing our dishes or just moving around, are actually really good for keeping those muscles strong.” Similarly, it’s important to keep our brain engaged and active, which can come in many forms. “It can come from staying socially connected. It can come from learning something new. But we want to think about exercising our brain and using our body, including thinking about ways to practice our fine motor skills … and if we have those things taken away and done for us, we’re not necessarily doing ourselves a service.”

Still, stresses Rowe, “There’s tremendous variability. And what we’re seeing is an increasing proportion of the older population that’s performing at very high levels who are kind of superagers.”

Enter the superagers…

Rogalski, through her research as part of the ongoing, multidisciplinary SuperAging Research Initiative, is looking at evidence from biologic, family history and lifestyle perspectives in order to learn what makes certain people seem to barely age, at least cognitively.

“What we’ve seen is that superagers, biologically, seem to look different. Their brains actually look more like 50 to 60 year olds than they do like 80 year olds,” she says, adding that their rate of shrinkage is slower than that of average 80-year-olds.

“So they seem to be resisting that thinning of the outer layer of the brain, or the cortex, and when we measure it using really precise tools, we see that the superager brains actually don’t show any shrinkage relative to the 50- to 60-year-olds,” she says. In fact, there’s a region of the brain called the anterior cingulate cortex (ACC)—which has a role in motivation, decision-making, and emotional and situational cues—that’s thicker in the superagers than it is in the 50- to 60-year-olds. They’ve also discovered an abundance of a neuron called von Economo neurons, helping scientists to have a “biologic pathway” for understanding superagers.

Years ago, Rowe tells Fortune, he ran a research network that studied “successful aging” at Harvard University. In one study, he followed a group of 75-year-olds for six years, testing them physically and cognitively over that period. “At the end, 25% had not changed, 50% had gotten much worse and the other kind of stayed in the middle,” says Rowe, noting that those who did the best, the superagers, shared certain lifestyle characteristics, including not living alone, educational attainment, and financial security.

It underscores how, were you to gather a bunch of 80-year-olds today to assess their cognitively abilities, you’d get mixed results: Probably a couple with dementia, a superager or two, and others who are in between. That’s not only due to people’s brains changing at different rates, but also the difference in lifestyles, genetics, and other factors.

Bottom line, says Rowe, who points out that he himself is 80, “I don’t think we can talk about an average with any meaningful validity when we are trying to reduce that to a decision about a person. I don’t think we can ascribe an average of an 80-year-old to an individual.”