Whether crossword puzzles, sudokus and other brain teasers actually keep your brain in shape, has not been well-established. However, lack of education is a strong predictor of cognitive decline. The more you've tried to learn, the better you'll be at mental sit-ups in old age. The key may be tackling something new; the challenge of the unknown is likely more beneficial than putting together the same jigsaw puzzle over and over again.

http://www.livescience.com/health/090423-sharp-mind-1.html