The document provides 20 tips for preventing dementia that experts recommend starting in one's 30s and 40s. Some of the key tips include volunteering, taking up hobbies like dancing, gardening, walking, reading, playing games, learning a musical instrument or new language, traveling, meditating, getting enough sleep, and eating a diet high in omega-3s and produce while socializing with family and friends. The goal is to engage in mentally and socially stimulating activities to exercise the brain throughout life.