How quickly are you growing old?

Feel like you’re 40 years old going on 60? Or maybe, 40 going on 21?

Age may be just a number, but medical experts increasingly are saying it might not always be the right number to gauge your health.

Everybody grows older at a different pace, according to a recent study that found the processes of aging can begin fairly early in life. The study calculated the aging rate of 954 men and women—taking various measurements of their bodies’ health—when they were each 26, 32 and 38 in chronological years. By analyzing how these measures changed over time, the researchers were able to see who aged faster and who slower than normal.

The aim of the research is to be able eventually to identify signs of premature aging before it becomes evident years or decades later in chronic diseases such as cardiovascular disease, diabetes or kidney and lung impairment. “Intervention to reverse or delay the march toward age-related diseases must be scheduled while people are still young,” according to the study, published online last week in the Proceedings of the National Academy of Sciences.

Also, being able to measure aging in young people could allow scientists to test the effectiveness of antiaging therapies, such as calorie-restrictive diets, the study said.

To measure the pace of biological aging, which the study defined as the declining integrity of multiple organ systems, the researchers relied on 18 separate biomarkers. These ranged from common measures such as HDL-cholesterol levels and mean arterial blood pressure to more obscure ones like the length of telomeres—the protective caps on the ends of chromosomes that shorten with age.

Most of the study participants aged one biological year for each chronological year. Some, however, put on as much as three biological years for every one year, while others didn’t increase in biological age at all during the 12-year span the study surveyed. Using a subset of the biomarkers, the researchers calculated that at 38 years old, the participants’ biological ages ranged from 28 to 61.

Studies looking at biological age have been done before, but mainly in older people who already had age-related diseases. Earlier studies also generally took just a single reading that compared chronological with biological age and didn’t examine the pace of aging over time.

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