Healthy lifestyle, good genes, not getting cancer.. if the person is healthy they can keep energy level into their 70s atleast, grandfather was swimming, working on his car, reading, learning programming, was sharper then half the people I know in their 30s/20s.
Can not stand talking to some of my old university buddies: fat, zero energy, sit in front of tv after work.. weekend? mcdonalds with beer and or pot and back to TV.. wife's not better, spaming FB with their kids pictures while trying to break 200lb on their diet of junkfood