This is a fascinating area of study. As you will see in the comments section at the bottom of the article, a common question is which came first–the life purpose or the healthy brain? Whatever the answer, my approach (both with myself and my clients) is to nurture interests that cause a person to feel productive and peaceful in the short run. If these pursuits also contribute to increased brain health in the long run, all the better.
http://medicalxpress.com/news/2015-03-purpose-life-health-aging-brain.html