The researchers used the data set to tabulate the typical, desirable VO2 max for a healthy person at every age from 20 to 90, creating specific parameters for fitness age. The concept is simple enough, explains Ulrik Wisloff, the director of the K. G. Jebsen Center of Exercise in Medicine at the Norwegian University and the senior author of the study. “A 70-year-old man or woman who has the peak oxygen uptake of a 20-year-old has a fitness age of 20,” he says. He has seen just this combination during his research.

The researchers have used all of this data to create an online calculator that allows people to determine their VO2 max without going to a lab. You’ll need your waist measurement and your resting heart rate. To determine it, sit quietly for 10 minutes and check your pulse; count for 30 seconds, double the number and you have your resting heart rate. Plug these numbers, along with your age, sex and frequency and intensity of exercise, into the calculator, and you’ll learn your fitness age.

The results can be sobering. A 50-year-old man, for instance, who exercises moderately a few times a week, sports a 36-inch waist and a resting heart rate of 75 — not atypical values for healthy middle-aged men — will have a fitness age of 59. Thankfully, unwanted fitness years, unlike the chronological kind, can be erased, Dr. Wisloff says. Exercise more frequently or more intensely. Then replug your numbers and exult as your “age” declines. A youthful fitness age, Dr. Wisloff says, “is the single best predictor of current and future health.”

NY Times