What do you see when you look in the mirror? Someone who looks good for her age or someone who could use a trip to the fountain of youth ?

The leaders of a long-term health study pinpointed more than a dozen factors they think can explain why you're aging faster or slower than other people your age. Their findings were published in the Proceedings of the National Academy of Sciences.

A team of researchers led by Duke University's Daniel Belsky tracked nearly 1,000 people born in 1972 or 1973 in Dunedin, New Zealand, from the age of 26 to age 38. They calculated the participants' biological age at the end of the period based on 18 factors that have been linked to aging.

These included cholesterol, body mass index, cardiorespiratory fitness, and the length of the telomeres — the caps at the end of chromosomes that shorten with age. They also measured gum health, the functions of the kidneys, lungs, liver, and the condition of the tiny blood vessels in the back of the eyes that mirror the brain's blood vessels.

Using these scores, researchers were able to calculate a biological age for each of the volunteers. They did this at age 26, 32 and 38 to determine the volunteers' pace of aging.

Most participants aged normally — at a rate of one biological year per one chronological year. But some aged as many as three biological years for every chronological year. The good news is that some participants didn't age much in a given year. They were the ones who looked young for their age.

Those who appeared to be biologically older than they were scored poorly on balance and coordination tests and had more problems with physical movements, like taking the stairs. When their photos were shown to college students, the participants rated as biologically older also looked older than their age to strangers.

The biological age for the 38-year-old subjects in the study ranged from younger than 30 to nearly 60.

This study was unusual in that it used young people. Most research on aging is done on older people. According to the researchers, the problem in those cases is that many of the study participants already have age-related diseases or physiological changes.

“We set out to measure aging in these relatively young people,” said 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.”

Belsky said that most people tend to think of aging as something that happens later on in life, but in this study, signs of aging were apparent in people from ages 26 to 38.

Though people often think aging (or not aging) is genetic, that isn't always the case. Research involving twins shows that only about 20 percent of aging can be linked to genetics, said Belsky. “There’s a great deal of environmental influence,” he said.

“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, professor of psychology and neuroscience at Duke.

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Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.