Even those who started exercise late saw benefit -- previously sedentary 85-year-olds who got a move on had three-year survival rates double that of elder couch potatoes.
Nor did it take intense training to qualify as active. It seems that walking four hours a week had as much benefit (in terms of survival, at least) as more vigorous activity.
So keep moving! Me, I intend to still be continuing my karate training into my 80s. And 90s. And 100s.