Study after study confirms that, if there is one salient answer to the question of how to feel younger, fitness is it, she explains in a chapter aptly titled "The Sweaty Truth." One compelling example, among many, is a study that found over the course of eight weeks of strength training, women between the ages of 87 and 96 tripled their muscle strength and grew the size of their muscles by 10 percent, she writes. Another study found that middle-aged men and women who exercised twice a week and ate a healthy diet cut their risk of Alzheimer's disease in half. Aging, Kessler contends, may be less a function of growing old than becoming inactive.
Kessler stresses integrating movement into your life, because even one hour of vigorous cardio at the gym isn't enough to reverse eight hours of sitting at a desk all day. For her part, she might start her day with a jog and end it with yoga. And in between, she sets her smartphone to buzz every hour, reminding her to move, be it chasing the cat or doing 50 jumping jacks. Her social life has changed, too. Instead of socializing over meals, she meets friends for walks or bike rides. And she may resume, two or three times a year, a 14-day detox diet that she writes left her feeling "unburdened, empowered, upbeat. And, yes, youthful."
[Read: How to Break Your Sitting Habit.]
And all of these efforts add up. "It is the accumulation of small decisions over a long period of time that keeps us youthful," she says.
Plus, one's attitude about aging correlates directly with how well we age. Kessler cites a Yale study that found that the "perceptions a person held about aging had more impact on how long he or she lived than did their blood pressure, cholesterol level, or whether they were smokers," she writes. "Regardless of age, gender, socioeconomic status, loneliness, or – get this – the actual state of their health, the men and women with positive views on aging lived seven and a half years longer than those who bought into the negative stereotypes."
[Read: 10 Tips to Lighten Up and Laugh.]
With that in mind, Kessler says she's avoiding peers who talk about slowing down: "Now is the time to rev up." And she brims with an invigoration when she imagines the possibility of this new kind of old age. "If you had both the experience of having lived that long and the vitality and energy because you are biologically youthful, consider that. I mean, how powerful would that be?"
After a year of investigating the latest fads and facts around healthy aging, and adopting the best recommendations from science and those she trusts, has Kessler gotten any younger? Yes. Her biological age, she says, "appears to be somewhere between 45 and 48."
So perhaps there is a Fountain of Youth after all. Seekers just have to reorient their compass, and point it right back at themselves.