If you want to know your true age, don’t look at the calendar, because counting the years  could give you a widely inaccurate result, a new study finds.

Duke University researchers calculated the biological age of nearly 1,000 adults age 38 and found that their biological ages ranged from 28 to 61.

Biological age is a measure of how well your body is functioning relative to your actual calendar age.

The researchers used data from the Dunedin Study, a landmark longitudinal project that has tracked more than a thousand people born in 1972-73 in the same town from birth to the present.

The team measured the function of subjects' kidneys, liver, lungs, metabolic and immune systems, as well as cholesterol, cardiorespiratory fitness, and the length of the telomeres -- protective caps at the end of chromosomes that have been found to shorten with age. They also looked at dental health and the condition of the tiny blood vessels at the back of the eyes, which are a proxy for the brain’s blood vessels.

Not only did they find a wide variance in age, but they also discovered that individuals, although born within months of each other, not only had widely varying biological ages, but also were aging at different rates, with those of an advanced biological age aging even faster.