People age differently. This is no more apparent than checking out people your age on Facebook and wondering why they look so much older than you, even though you were born in the same year.

According to a paper published in the Proceedings of the National Academy of Sciences, researchers from the U.S., UK, Israel, and New Zealand, there are 18 biological measures that combine to determine how fast or how slow a person is aging.

Researchers took the data from the Dunedin Study, which tracked more than a thousand people that were born in 1972-73 in the same town. The study took readings of blood pressure and liver function regularly, while also conducting interviews and other assessments.

"We set out to measure aging in these relatively young people," said first author Dan Belsky, an assistant professor of geriatrics in Duke University's Center for Aging. "Most studies of aging look at seniors, but if we want to be able to prevent age-related disease, we're going to have to start studying aging in young people."

Organs are capable of showing aging in people, the same way that you can see it in your skin or hair or feel it in your joints. Researchers looked at the functions of kidneys, liver, lungs, metabolic and immune systems, as well as HDL cholesterol, cardiorespiratory fitness, lung function and the length of the telomeres -- protective caps at the end of chromosomes that have been found to shorten with age. They also checked dental health and the blood vessels behind the eyes.

All of these factors led researchers to declare a “biological age” for all the participants in the study. The 38-year-olds “biological age” ranged from under 30 to nearly 60. As expected, those the team deemed older than their birth age were actually aging at a slightly faster rate than those with a lower biological age.

Though most of the subjects aged the same chronologically as they did biologically, others aged three times faster biologically than they did chronologically. There were some, however, that aged zero years biologically for every year they aged chronologically, meaning they essentially stayed younger than their age.

"That gives us some hope that medicine might be able to slow aging and give people more healthy active years," said senior author Terrie Moffitt, the Nannerl O. Keohane professor of psychology and neuroscience at Duke University. Researchers asked graduate students at Duke to assess each facial photo of the subjects and say how old or young they appeared. The biologically older subjects also appeared older to the students.

The ultimate goal of the paper is to intervene with the aging process itself.

"As we get older, our risk grows for all kinds of different diseases," Belsky said. "To prevent multiple diseases simultaneously, aging itself has to be the target. Otherwise, it's a game of whack-a-mole."