In a study of elderly individuals over a span of nine years, it was found that those who were active and walked daily maintained more brain mass than those who did not.  With the brain, according to the article from CBS News, size does matter.  The brain shrinks with age, and shrinkage can lead to dementia later in life — but the brains of those with active lifestyles did not shrink at the same rate as those with more sedentary lifestyles.

Other things that help protect the brain besides exercise:  eating leafy greens and fruits, exercising your brain (especially during middle age) with crossword puzzles and other things that stimulate the creation of new connections in the brain, remaining socially engaged, and having a pet.