How young is your brain? Try this self-test: Stand on one leg and close your eyes. The longer you can stand without losing your balance, the younger your brain is—15 seconds is very good if you are 45 or older. Have someone spot you if there's any chance you might not recover your equilibrium in time to avoid a spill.

To help keep your brain young and prevent memory loss, avoid living on autopilot—doing the same things day after day. If you stretch yourself mentally, you'll actually avoid brain shrinkage. The classic way to do that is to learn something new—whether it's speaking Spanish or playing the harmonica. Like muscles, your brain grows when it's working beyond its normal routine.

Another way to build your brain is by "testing at your threshold." Let's say you can always do Wednesday's crossword puzzle, but you get barely half of Sunday's answers. The best thing for your brain is to continue taking a whack at Sunday's puzzle. Just as an athlete becomes faster or stronger by training to attain goals that are just out of reach, you can train your brain to stay smarter and sharper.