But those who start younger reap the greatest rewards, adds Kramer, a physical fitness nut who has even climbed Alaska's Denali (also known as Mount McKinley), the highest peak in North America.
If you want to keep your senses, he says, the evidence is clear: "Get off the couch, no matter how old you are."
"There's no reason not to start if you're older, and there's no reason not to start earlier if you're younger," he says. "We've been doing this kind of work for years, and 20-year-olds always say to me, 'Well, what does it matter? I can always wait until I'm 60.'
"My reply is the effects tend to be larger if you start younger. So if you plan to be around when you're 70, it might be a good idea to start now."
Another study led by Kramer, which will be published in the March issue of Psychological Science, revealed some similar results. It found:
Exercise programs involving both aerobic exercise and strength training produced better results on cognitive abilities than either one alone. That suggests that the old rule of walking 30 minutes a day, three days a week, may not offer as much protection against mental decline as a more vigorous routine.
Older adults benefit more than younger people because age-related declines are more pronounced.
The magnetic imaging study will be published in the February issue of the Journal of Gerontology: Medical Sciences. The studies were funded by the National Institute on Aging and the New York-based Institute for the Study of Aging.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.