To officially calculate your fitness age, you’ll need to fill out a questionnaire devised by exercise scientist Ulrik Wisløff at worldfitnesslevel.org.

Here, Wisløff walks senior digital editor Michael Rodio, a casual lifter and erstwhile CrossFitter, through each question. (And don't forget to read our in-depth feature on 5 ways to maintain the body of a 20-year-old for a deeper dive into the importance of determining your fitness age and the best ways to lower it.)

Step 1: Where are you from? Ethnicity? Highest level of education?

Wisløff says: “We use that only for research purposes so we can compare Norway, USA, Japan, etc., and different ethnicities—nothing of this is used in the algorithm.”
Rodio's take: I type in “Brooklyn.”

Step 2: Gender, age, height, weight.

Wisløff says: “Height/weight is just for calculating BMI, which goes into the algorithm. But gender matters a lot—women’s values tend to be about 20% lower.”
Rodio's take: I’m 26, 5'9", 175 pounds. I enter twice, because I accidentally type in my weight in kilograms the first time.

Step 3: What’s your maximum heart rate?

Wisløff says: “It’s a common means of denoting intensity for endurance training.”
Rodio's take: I had no idea, but the site computes it for me. (It’s 196 bpm, apparently.)

Step 4: How often, how long, and how intense is your exercise?

Wisløff says: “All these factors matter in a balanced way, but exercise intensity is the most indicative of fitness age.”
Rodio's take: I pick “little hard breathing and sweating,” because there’s no option for “vigorous swearing or crying.”

Step 5: What’s your waistline? What’s your resting pulse?

Wisløff says: “A low resting heartbeat is the sign of a fit heart—world-class endurance athletes use it to see if they’re ready for their next exercise session—but we do know that it’s not enough to predict fitness on its own. Hydration can sway it, for example, so make sure you’re hydrated when you take your measurement.”
Rodio's take: My belt suggests a 31-inch waistline. My Fitbit says 55 bpm.

The takeaway

Even though I’m 26, with an expected VO2 max of 53, I have the fitness age of someone under 20 years old, with an actual VO2 max of 60.

Ready to take Wisløff's questionnaire? Find out your fitness age at worldfitnesslevel.org—and read our feature on how to lower yours.