A recent NPR story about staving off dementia in old age included many of the usual suggestions:

• moderate aerobic exercise
• mental exercises (Sudoku and the like)
• learning a new language
• staying social

One neuroscientist had an even better suggestion, though:

…the best advice might be to join a book group that walks and drinks red wine while talking about the book.

It covers everything. It’s also less likely to involve breaking a sweat, which is always good.