A detailed study of biological ageing has revealed that some individuals grow older three times faster than their birthdays suggest.
For others blessed with evergreen genes or environmental influences, time literally appears to stand still.
Among a group of 38-year-olds taking part in the research, biological age was found to range from 28 to 61.
US scientists drew up a panel of 18 measures to determine the speed at which a person is ageing.
They included tests of kidney, liver, lung and immune system function as well as assessments of metabolism, cholesterol, heart health, lung function, and the length of telomeres - protective caps on the ends of chromosomes.
Researchers also looked at the condition of tiny capillaries at the back of the eye, which provide a glimpse of the state of blood vessels in the brain.
"We set out to measure ageing in these relatively young people," said lead scientist Dr Dan Belsky, from the Duke University Center for Ageing in North Carolina.
"Most studies of ageing look at seniors, but if we want to be able to prevent age-related disease, we're going to have to start studying ageing in young people."
The 871 participants had all been enrolled in the Dunedin Study, a major investigation tracking the health of about 1000 New Zealanders born in the town of Dunedin in 1972-73.
Of the original group, 30 had died by the age of 38 due to serious diseases such as cancer, accidents, suicides and drug overdoses.
The 18 biomarkers were measured when the volunteers were 26, then again when they were 32, and finally at 38. Combining the measurements allowed scientists to determine each individual's pace of ageing.
For most participants, chronological age and biological age kept at roughly the same pace.
But some were found to have bodies ageing as fast as three years per chronological year, while others aged at zero years per year - effectively, not getting older at all biologically.
Participants with more advanced biological ageing scored worse in tests of balance, co-ordination and solving unfamiliar problems typically given to people over 60.
In addition, biologically older individuals reported more difficulty undertaking physical tasks such as walking up stairs. And their faces seemed older, according to college students asked to rate their photos.
"Already, before mid-life, individuals who were ageing more rapidly were less physically able, showed cognitive decline and brain ageing, self-reported worse health, and looked older," the scientists wrote in their findings.
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