Bob Heins, 71
Heins is living proof that older muscles can still be strong. A dentist and serious competitive triathlete in Plattsburgh, New York, Heins holds national and age-group course records in the Ironman triathlon. "Some of the older guys out there are doing an under-four-hour marathon," he says of his fellow triathletes. "I know plenty of 30-year-olds who can’t do that."