Hawaii's population is aging faster than almost anywhere else in the country, and staying curious is emerging as a simple defense against depression and mental decline in retirees.

• Hawaii has more than 310,000 residents over 65, and a third of retirees there say they feel depressed.
• The main cause is loss of purpose: after 40 years of a job setting the daily routine, retirement can leave people listless.
• "Proactive curiosity" — trying something completely new as a true beginner — builds new brain pathways and delivers a dopamine boost.
• Learning a new recipe counts, as does a language, sport, or dance class; doing it alongside younger people works even better.
• New activities are usually social, which also fights the isolation that fuels depression.

Outlook: With September marked as Healthy Aging Month, the push is for retirees to stay fascinated rather than worry about staying young.