It may not be quite that simple but there is enough evidence accumulating to believe that staying active in our bodies almost magically boosts our mental abilities and helps us stay sharp indefinitely.
Things To Exercise Your Brain
(From Psychology, by Hockenbery, Hockenbery and Nolan).
Get regular aerobic exercise.
Seek out social interaction, don't hide in your room or apartment.
Learn to play a musical instrument. An iPhone doesn't count.
Take a class in a field outside your college major or in a new area.
Read widely - books, journals.
Try puzzles of any kind - word, number, mazes, matching
Unplug your television set.