The results of studies about Alzheimer's and older age dementia of all types have shown mixed results regarding the benefits of exercise. The studies to date had been pretty small undertakings and had not had very long follow-ups. A new study out -- published in the Annals of Internal Medicine, Feb. 5, 2013 --- shows that there does seem to be a significant risk reduction for developing dementia by beginning an exercise program in your later 40's.
The amount of exercise is not extreme either. The amount used in the study was 150 minutes of moderate exercise a week, which would amount to a brisk walk for about 20 minutes every day. If you start that in your late 40's your risk for developing any kind of dementia drops by 36% over those who aren't exercising. So in addition to reducing your risks from heart disease and cancer you can add dementia to the growing list of chronic health problems avoided by getting off your couch and moving for 20 minutes a day.