No key. There isn't a single key that is great for all, but a universal truth includes maintaining a balance. Healthful eating of foods that lack processing and ingredients that you cannot pronounce along with a daily exercise program assists. The mind has to be nourished with new information, achievements and assisting others to be strong. Smile and laugh often and keep your mind limber and open.
Holistic approach. Positive and supportive friendships, active social life, healthy family relationships.
Lifestyle. Eat healthfully w at least 5 servings of vegetables & 4 of fruit /day (organic is best). Increase nutritional fiber. Avoid refined sugar & junk carbohydrates. Avoid sweetened juices, soda & caffeinated beverages. Watch portion sizes. No tobacco, illicit drugs & low to no alcohol use. Aim for 7.5 to 8 hrs sleep /night. Hydrate w at least 64 oz of water / day. Maintain good hygiene. Daily physical.