The Washington Post has an article today about lifestyle exercises to slow the brain's aging. The rate of slowing down with older people is a combination of genetics, environmental factors, and lifestyle. I don't have any doubt that keeping your mind active-- the woman described in the article is taking Italian classes and doing crossword puzzles-- will help with the third aspect.
The article does a good job keeping this work in perspective: no single person, for example, ever defeated Alzheimer's disease by doing crosswords, but in on average the disease hits later among people with active mental lives. (And people with early Alzheimer's can use mental exercises to improve their mental function. )
This idea with respect to Alzheimer's has been around for some time. I think what is new is that this appears to be a good idea for everyone.
Very interesting stuff.