"Really? You look much younger than that!" may be more than just a complimentary cliché we offer to someone; people do actually age at different rates, a new study suggests.

An international team of researchers looked at biological markers for the effects of aging, using a study of the health of around 1,000 men and women in New Zealand who have been monitored from their years of birth of 1972 and 1973, up to the present.

They found that the "biological age" of the participants in 2011, when they were 38 — as exhibited by the state of their organs, their immune systems, their heart health and their chromosomes — ranged from as young as 30 to as old as 60.

A comparison of the same health measurements taken at ages 26 and 32 showed how rapidly each person was aging, the researchers said in their study published in the Proceedings of the National Academy of Sciences.

"What we found is a clear relationship between looking older on the outside and aging faster on the inside," says study author Daniel Belsky, a professor of medicine at Duke University. "And also that it's possible to measure the kind of aging process in young people that we usually only look for in old people."

While the "biological" aging rate for most was, as expected, around one year per calendar year, some were found to be aging as much as three years per calendar year, while others showed almost zero years per date year.

Biological age increases in sync with chronological age for most young adults, the research team says, but genetic and environmental factors can result in biological aging occurring much faster — or much slower — than a person's birth date might predict.

While the study couldn't answer the question as to why some people age faster than others, further research could yield some answers, Belsky says.

"I think the three general categories of factors that people think about in research on aging are genetics, early-life experiences like chronic stress or trauma, and lifestyle factors like diet and exercise," he explains.

Determining the rate at which different people age could be highly useful, he adds, because that could allow scientists to test the effectiveness of therapies directly aimed at preventing disease by slowing the aging process.

"Currently, such evaluations require very long follow-up times because we have to wait and see if people develop chronic disease or die prematurely," he says.

The ability to directly measure the rate of aging would allow the testing of age-slowing therapies in real time, he notes.

Photo: Omer Wazir | Flickr