If you're surprised by how much older some of your school peers look every time you see them, while others tend to look exactly the same, your eyes probably aren't failing you.
A new study from Duke University shows how people born within one year of each other age at phenomenally different rates, with some being biologically on the verge of retirement when they're just 38.
The researchers looked at how 18 different physical traits varied between almost 1,000 people when they reached 26, 32 and 38. These included weight, kidney function, cardio fitness and gum health.
Those who appeared to be more advanced in biological ageing also scored worse on tests typically given to people over 60, such as tests of balance and coordination and solving unfamiliar problems. They also reported having more difficulties with physical functioning than their peers, including maintaining a firm grip on objects – a factor influencing the strength of a handshake.
While most people aged at a rate reflective of their real age, for some people three biological years passed for every actual year. For some others, they didn't age at all.
The results are published in the journal Proceedings of the National Academy of Sciences.
While we often blame quick ageing on factors such as stress and bad diet, what it actually comes down to is not clear.
Finding that out will require further work, the researchers say.
They do know, however, that genetics play only a minor role in determining rate of ageing. Previous studies of twins has revealed how just 20 per cent of ageing can be attributed to genes, with the rest being environmentally driven.
"That gives us some hope that medicine might be able to slow ageing and give people more healthy active years," said Terrie Moffitt, one of the study's lead authors.