Your brain’s health may be the most powerful indicator of how long you will live. It is crucial to whether that life will be rich and satisfying from youth well into old age, or something substantially less rewarding, and for less time.

A car driven wisely, fueled with high-quality gasoline, given regular oil changes, and repaired with new parts as old ones wear out, is likely to last longer than one that’s abused or neglected. Likewise, the easiest way to have a healthy brain in middle age and beyond is to follow good physical and mental habits.

But what if a person comes late to repairs, like the owner of a car that rusts for years on blocks or runs too long on dirty oil? The car owner can always swap out the engine. You, on the other hand, have only one brain, basically composed of the same neurons you were born with, plus a few added to some narrowly specific areas. Once they’ve begun to deteriorate, can they be saved—or even made stronger?

Tale of two rats

Brain researcher Marian Diamond is certain it’s never too late to improve your brain function, and here’s why.

In the 1960s, Diamond compared two groups of lab rats. The first group was confined to the equivalent of a gray isolation cell in a maximum-security prison. They ate simple rations to keep them alive from day to day, but their brains received little stimulation. No rat games, no rat puzzles, no rat get-togethers to break the boredom. She enrolled the second group in a version of rat school, complete with recess. They had toys and balls for play, challenging mazes to explore, exercise equipment to get blood pumping to their muscles and their neurons, and best of all, other rats to share their experiences. When she pitted the two in timed contests in which they ran the same mazes, the rats that had lived in the mentally and physically invigorating environment performed much better.

Diamond then did what she could not do to humans in a similar experiment. She put both winners and losers under the knife to examine their brains. (Life’s not fair, especially for a rodent.) Rats that had enjoyed the richer learning environment and had won the maze races exhibited markedly different brains from those in the control group. Their cerebral cortices—the outer, wrinkled shells that are home to neural pathways that make sense of the world—were thicker than those of the unstimulated rats.

The enriched-brain rats had more neural connections, a sign of greater mental activity. And they had more blood vessels to carry vital oxygen to keep those connections firing at peak efficiency. Diamond had gathered concrete evidence that what goes on in the mind manifests itself in the physical state of the brain. Learning strengthens the organ of the brain just as exercise strengthens muscles in the legs, arms, and abdomen.

As revealing as Diamond’s research was, it had a twist: She didn’t experiment on young rats. She chose to work with rats in middle age and older, equal to ages between 60 and 90 in humans. Old rats had brains they could reshape in response to new experiences, a condition known as plasticity.

That’s good news, and not just for rats. The structure of the brain remains remarkably similar for all mammals. What works for mice, dogs, horses, and monkeys works for humans as well. Diamond took comfort in her findings that the brain can change at any age. Older brains take longer to respond to healthy living, but they do respond. “We’re saying that if you use your brain, you can change it as much as a younger brain,” she said.

Beating the clock

Time works against the brain in three ways. When the brain reacts negatively to aging, it does so through disease, disuse, and physical changes associated with aging itself. Diseases become more common with age, and many attack the brain. They range from strokes, which kill brain cells by cutting off the blood supply, to cancerous tumors and dementia.

Disuse causes neglected neural connections to fade, eventually severing connections entirely. Who, in middle or old age, hasn’t forgotten vast chunks of high school trigonometry, if never used since age 18, or become rusty at chess after years without a challenging opponent?

Finally, aging itself prunes some of the brain’s neural thickets, eliminating some neurons and leaving the remaining ones susceptible to the cumulative effects of a lifetime of exposure to toxins and other natural chemical agents.

And yet, practically everyone knows a person who has lived to 80, 90, or beyond while remaining mentally healthy. The brain of a healthy senior citizen processes information more slowly than a youthful one, but once it has learned something, it keeps it as treasure to be used again and again.

Change at any age

All parts of the brain, not just the ones related to higher forms of thinking, can be improved through stimulating challenges—at any age. Someone who wants to improve balance can take up tai chi at age 30 or 90. Wii bowling improves eye-hand coordination and the ability to focus attention just as well for senior citizens as for teenagers.

In fact, exercise by the elderly has been shown to diminish the risk of falls, increase mobility, and possibly fight dementia. Yet only about one in eight Americans between ages 65 and 74, and one in 16 over age 75, reports performing robust physical activity for at least 20 minutes three times a week, the recommended minimum.

The brain’s plasticity reveals much about its amazing structure. It is the most complicated object we’ve yet discovered in the universe, composed of billions of independent units that work together in remarkably complex symphonies that manage to comprehend the world; process, store, and retrieve information; and use that information to decide how to interact with the world. Each new experience changes the brain’s physical makeup, so that by the time you finish reading this page, your brain will be slightly different from your brain at the time you began with the page’s first word.

Repeating familiar experiences is good, up to a point: Practicing an old favorite on the guitar changes the brain in ways that improve future performances. But the best stimulation for the brain, young or old, is novelty.

Even rats given a nest full of colorful toys find them boring after a while, because playing with them fires the same well-worn neural pathways and takes less and less mental effort. New experiences—new ways to learn—keep the brain more robust at any age because they spur new connections among the brain’s neural circuitry. And the more connections the brain has, the better able it will be to stand up to the changes brought about by normal aging and disease.

Portions of this material appeared in National Geographic Complete Guide to Brain Health by Michael Sweeney. Copyright (c) 2013 National Geographic Society.