My husband, at 74, is the baby of his bridge group, which includes a woman of 85 and a man of 89. This challenging game demands an excellent memory (for bids, cards played, rules and so on) and an ability to think strategically and read subtle psychological cues. Never having had a head for cards, I continue to be amazed by the mental agility of these septuagenarians and octogenarians.

　　The brain, like every other part of the body, changes with age, and those changes can impede clear thinking and memory. Yet many older people seem to remain sharp as a tack well into their 80s and beyond.

　　Although their pace may have slowed, they continue to work, travel, attend plays and concerts, play cards and board games, study foreign languages, design buildings, work with computers, write books, do puzzles, knit or perform other mentally challenging tasks that can befuddle people much younger.

　　Several studies of normal aging have found that higher levels of educational attainment were associated with slower cognitive and functional decline. But brain stimulation does not have to stop with the diploma. Better-educated people may go on to choose more intellectually demanding occupations and pursue brain-stimulating hobbies, resulting in a form of lifelong learning.

　　You’re doing the same thing over and over again, without introducing new mental challenges, it won’t be beneficial. Thus, as with muscles, it’s “use it or lose it.” The brain requires continued stresses to maintain or enhance its strength.

　　Long-term studies in other countries, including Sweden and China, have also found that continued social interactions helped protect against dementia. The more extensive an older person’s social network, the better the brain is likely to work. Especially helpful are productive or mentally stimulating activities pursued with other people, like community gardening, taking classes, volunteering or participating in a play-reading group.

　　Perhaps the most direct route to a fit mind is through a fit body. Just walking fast for 30 to 60 minutes several times a week can help. Even those who start exercising in their 60s cut their risk of dementia in half.