Ageing rates vary widely, says study.
The report, in Proceedings of the National Academy of Sciences, tracked traits such as weight, kidney function and gum health.
Some of the 38-year-olds were ageing so badly that their "biological age" was on the cusp of retirement.
The team said the next step was to discover what was affecting the pace of ageing.
The international research group followed 954 people from the same town in New Zealand who were all born in 1972-73.
The scientists looked at 18 different ageing-related traits when the group turned 26, 32 and 38 years old.
The analysis showed that at the age of 38, the people's biological ages ranged from the late-20s to those who were nearly 60.
"They look rough, they look lacking in vitality," said Prof Terrie Moffitt from Duke University in the US.
The study said some people had almost stopped ageing during the period of the study, while others were gaining nearly three years of biological age for every twelve months that passed.
People with older biological ages tended to do worse in tests of brain function and had a weaker grip.
Most people's biological age was within a few years of their chronological age. It is unclear how the pace of biological ageing changes through life with these measures.
Prof Moffitt told the BBC: "Any area of life where we currently use chronological age is faulty, if we knew more about biological age we could be more fair and egalitarian."
She argued the retirement age may be unfair for those "working at their peak" who then had to retire.
The researchers said it was unexpected to find such differences so early, but that the findings could help trial methods for slowing the pace of ageing and ultimately have implications for medicine.
She added: "Eventually if we really want to slow the process of ageing to prevent the onset of disease we're going to have to intervene with young people."
Dr Andrea Danese from King's College London said: "I think it is surprising you can detect these changes in young people.
"This is the first step to detect factors that influence the rate of ageing... very early prevention [of diseases] may be one of the things we will be able to measure better."