From Scientific American:
The mind is not as agile as it once was, even at the ripe old age of 34. Names elude me, statistics slip away, memory fades. This is just the first step on a long journey into senescence; and by 74, if I make it that far, I might remember practically nothing. That age is the average of a cohort of 2,802 seniors who recently participated in a long-term study to see if anything can be done to reverse this age-related mind decline. The good news: there is.
Sherry Willis of Pennsylvania State University led a team of scientists that followed this group of adults, aged 65 and older, still living independently between 1998 and 2004. The seniors came from all walks of life, races, and parts of the country, including Birmingham, Ala., Detroit, Boston and other major cities. They all had one thing in common when the study commenced: no signs of cognitive impairment.
More here.