A little over a year ago, I took a series of private classes at True Pilates, a no-frills establishment in New York City. Though I enjoyed the sessions, I found myself distracted by something else going on in a corner of the room: a group of older women in leotards—many of them, it appeared, were in their seventies and beyond—doing somersaults and splits; some were even lifting themselves up on rings suspended from the ceiling and doing lithe, athletic flips. I wondered: ex–circus performers? Former Olympians? How could senior citizens be so extraordinarily bendy? Even after I abandoned Pilates and moved on to barre, then yoga, then spinning—sampling the city dweller’s smorgasbord of fitness options without finding one that wholly satisfied me—I couldn’t stop thinking about those ladies. Did they know something I didn’t? Was basic, old-school gymnastics—something that, to this day, can conjure up shudders of junior-high dread in me—the secret to remaining forever fit?