Forget the birth certificate. You are only as old as your VO2 max says you are.

You can’t change the year you were born, but you can change your “fitness age.” The best part? Fitness age is actually a better predictor of health and longevity than chronological age.

In order to calculate your fitness age, first you’ll need to calculate your VO2 max—which is not the same as your max heart rate, but is equally as useful. Your VO2 max measures how efficiently your body processes oxygen and is a great indicator of cardiovascular endurance.

Normally it’s best not to compare yourself to others, but your fitness age is determined by the VO2 max averages for your age group.

A VO2 max that is above average for your age group indicates that your fitness age is younger than your actual age. A 45-year-old woman, for example, might have a fitness age of 30 if her VO2 max is above average. Conversely, a VO2 max that is below average corresponds to an older fitness age.

Exercise often to live longer.

Calculating your fitness age is worthwhile because it can provide a glimpse into the future. A 2014 study published in Medicine & Science in Sports & Exercise found fitness age to be a significant predictor of life expectancy.

The study used data from 55,000 adults who had filled out lengthy health surveys dating back to the 1980s. Researchers were able to approximate each person’s fitness age based on how they answered the survey questions. The next (morbid but necessary) step was to check the death records.

The results were shocking. Researchers found that fitness age may be a better predictor of longevity than more commonly discussed risk factors like obesity, high blood pressure, and smoking. Adults with a VO2 max that was 15 percent or more below the average for their age group had an 82 percent higher chance of premature death. Their fitness age was considerably older than their chronological age. The older the fitness age, the higher the risk.

Luckily, you can turn back time—all you have to do is exercise