We all have friends who were born in the same year but look years younger (or older) than we do. Now researchers say that such perceptions aren’t just about outward appearances but about something deeper—the different pace at which each of us ages, and what that means for our health.

In a study published in the Proceedings of the National Academy of Sciences, scientists led by Daniel Belsky, an assistant professor of medicine at the Duke University School of Medicine’s division of geriatrics, describe a panel of 18 measures tested in 20- and 30-year olds that showed how quickly they are aging. The markers proved to be a good indicator of physiological age; they mirrored the biological effects of aging found in older people. But they were also good markers of physical age, meaning that those who aged faster also looked older, according to unbiased assessments by random people…