THERE’S A STORY THAT MANY PEOPLE TELL themselves about aging, and it goes something like this: As they get older, their physical health span declines precipitously when approaching the final decades of life. They lose speed, strength, reaction time, balance, stability, and more. These processes are subtle at first, though they accelerate with age. By 75, most people can expect a significant reduction in both muscle mass and physical-activity level—each of which cuts into their quality of life.