A simple urine test could measure how much our body has aged and even how long left we have to live.
Researchers have discovered that a substance indicating oxidative damage increases in urine as people get older.
The study, published in the journal Frontiers in Ageing Neuroscience, also describes a way to easily measure levels of the marker in human urine samples.
Researchers say the new marker potentially provides a method to measure how much our body has aged - our biological rather than chronological age.
And they believe it could help predict our risk of developing age-related disease and even our risk of death.
While everyone born in the same year has the same chronological age, the bodies of different people age at different rates.
Researchers said it means that, although the risk of many diseases increases with age, the link between our age in years and our health and lifespan is relatively loose.
Many people enjoy long lives, relatively free of disease, while others suffer chronic illness and premature death.
Some researchers consider normal ageing to be a disease, where our cells accumulate damage over time.
The rate of cellular damage can vary from person to person and may be dictated by genetics, lifestyle and the environment we live in.
The researchers said that cellular damage may be a more accurate indication of our biological age than the number of years since we were born.
And finding a way to measure biological age could help to predict the risk of developing age-related disease and even death.
One mechanism thought to underlie biological aging involves a molecule vital to human survival - oxygen - in what is called the free radical theory of ageing.
Dr Jian-Ping Cai, of the National Centre of Gerontology in China, said: "Oxygen by-products produced during normal metabolism can cause oxidative damage to biomolecules in cells, such as DNA and RNA.
"As we age, we suffer increasing oxidative damage, and so the levels of oxidative markers increase in our body."
One such marker, called 8-oxo-7,8-dihydroguanosine - or 8-oxoGsn for short, results from oxidation of a crucial molecule in our cells called RNA.
In previous studies in animals, the researchers found that 8-oxoGsn levels increase in urine with age.
To see if this is true for humans as well, the researchers measured 8-oxoGsn in urine samples from 1,228 Chinese residents aged from two-years-old to 90, using a rapid analysis technique called ultra-high-performance liquid chromatography.
Dr Cai said: "We found an age-dependent increase in urinary 8-oxoGsn in participants 21 years old and older.
"Therefore, urinary 8-oxoGsn is promising as a new marker of ageing."
Levels of 8-oxoGsn were roughly the same between men and women, except in post-menopausal women, who showed higher levels.
The researchers said that might be down to the decrease in oestrogen levels that happens during menopause, as oestrogen is known to have anti-oxidant effects.
The researchers said their rapid analysis technique could be useful for large-scale aging studies, as it can process urine samples from up to 10 participants per hour.
Dr Cai added: "Urinary 8-oxoGsn may reflect the real condition of our bodies better than our chronological age, and may help us to predict the risk of age-related diseases."