Using a simple test of gait speed, researchers were able to measure the ageing process.
Not only were slower walkers' bodies ageing more quickly - their faces looked older and they had smaller brains.
The international team said the findings were an "amazing surprise".
Doctors often measure gait speed to gauge overall health, particularly in the over-65s, because it is a good indicator of muscle strength, lung function, balance, spine strength and eyesight.
Slower walking speeds in old age have also been linked to a higher risk of dementia and decline.
'Problem sign'
In this study, of 1,000 people in New Zealand - born in the 1970s and followed to the age of 45 - the walking speed test was used much earlier, on adults in mid-life.
The study participants also had physical tests, brain function tests and brain scans, and during their childhood they had had cognitive tests every couple of years.
"This study found that a slow walk is a problem sign decades before old age," said Prof Terrie E Moffitt, lead author from King's College London and Duke University in the US.
Even at the age of 45, there was a wide variation in walking speeds with the fastest moving at 2m/s at top speed (without running).
In general, the slower walkers tended to show signs of "accelerated ageing" with their lungs, teeth and immune systems in worse shape than those who walked faster.
The more unexpected finding was that brain scans showed the slower walkers were more likely to have older-looking brains too.
And the researchers found they were able to predict the walking speed of 45-year-olds using the results of intelligence, language and motor skills tests from when they were three.
The children who grew up to be the slowest walkers (with a mean gait of 1.2m/s) had, on average, an IQ 12 points lower than those who were the fastest walkers (1.75m/s) 40 years later.
Lifestyle link
The international team of researchers, writing in JAMA Network Open, said the differences in health and IQ could be due to lifestyle choices or a reflection of some people having better health at the start of life.
But they suggest there are already signs in early life of who is going to fare better in health terms in later life.
The researchers said measuring walking speed at a younger age could be a way of testing treatments to slow human ageing.
A number of treatments, from low-calorie diets to taking the drug metformin, are currently being investigated.
It would also be an early indicator of brain and body health so people can make changes to their lifestyle while still young and healthy, the researchers said.
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