A new study suggests that the number of birthdays you've celebrated—your "chronological age"—is not an accurate indication of your "biological age." Rather, people age at vastly variable rates—some (unfortunately) faster than average, while others lag behind, lingering in youthfulness.

An international team of researchers followed 954 people from the same town in New Zealand, who were all born in 1972-73, for over a decade. At three different milestones—26, 32, and 38 years old—the participants were tested on 18 different aging-related traits, covering physical deterioration and mental acuity. They found that by just 38 years old, the signs of variable aging were already stark, with people's biological ages ranging from the late 20s to nearly 60. While most people were aging as expected, the extreme outliers had aged either almost not at all over the 10 year period, or were gaining nearly three years of biological age for every twelve months that passed.

Researchers were surprised by the results, published in Proceedings of the National Academy of Sciences. "They look rough, they look lacking in vitality," Prof. Terrie Moffitt from Duke University, told the BBC bluntly of the extreme agers. Although discrepancies in physical and mental health have long been recognized later in life, the study's major revelation was how early in life these differences were detectable.

Although the cause of variable aging is not yet known, a better understanding of the process could have huge implications for scientists hoping to develop life-prolonging medicines. "Eventually if we really want to slow the process of aging to prevent the onset of disease, we're going to have to intervene with young people," Moffitt said. Which means, if you aren't lucky enough to age as slowly as you'd like, science could soon help you retain that youthful feeling anyway.