Recent studies concur, demonstrating that early retirement comes with risks, especially to your brain. A few months ago researchers from the RAND Center for the Study of Aging and the University of Michigan published a study showing that cognitive performance levels drop earlier in countries that have younger retirement ages.

The researchers propose several reasons this may be so. Retirement often takes you away from an engaging social environment, and social interaction is thought to be necessary in establishing "cognitive reserve," a brain-backup system that allows you to function normally despite age-related brain damage. Once you retire, you're also less motivated to participate in mentally stimulating activities. For instance, if you no longer need to read the business section to study your competition, you may not read the paper at all. Both of these are variations on the "Use it or lose it" theory of cognition.

An active retirement boosts cognitive health. — Photo by Ben Hupfer/Corbis

Instead of treating retirement as a time to take it easy, I propose a different path. In Japan they call it ikigai, which means "the reason for which we wake up in the morning." Finding that reason for living is critical, especially in the United States, where so much of our ikigai seems tied to our careers.

First, identify activities that you enjoy and that help maintain your cognitive health. The best activities combine social engagement, physical activity, and intellectual stimulation. Dancing is one such activity.

Begin exploring these brain-stimulating hobbies before you retire. Just as you need a financial plan for retirement, you need a mental plan, too.