How old are you?

But recently,researchers have identified some particularly good indicators of time’s largely hidden toll on our bodies and how fast it’s increasing. Developing an “easy way to measure biological age will have a wide array of applications in prediction and prevention of age-related diseases,” says Dr Kang Zhang,founding director of the Institute for Genomic Medicine at the University of California.

Experts on ageing generally agree that acceptable biomarkers of ageing should foretell the remaining life span of a middle-aged person more accurately than chronological age does.

Proposed biomarkers of ageing haven’t yet convincingly cleared these hurdles,he added. But some provocatively telling ones have come to light.

In a 2010 study,medical records of 4,097 women,collected over two decades,were analysed to sift out 13 factors that best predicted future mortality. Taken together,the factors “characterise the clinical presentation of healthy ageing” in older women,the study concluded.

This year,Zhang and his colleagues reported that a kind of molecular ageing clock is embedded in our genomes whose speed can be measured via blood testing. The moving parts of the clock consist of chemical tags on DNA molecules. The researchers found that the patterns of the tags,called epigenetic markers,predictably change with age. Collectively,these tags spell out a “signature for age”,meaning these markers may be less muddied by confounders than other factors tied to ageing.

Of the markers,71 most indicative of chronological age were selected to measure the speed at which people are growing old. That was calculated by comparing a subject’s epigenetic tags to the norm for his or her age—a 40-year-old whose pattern closely resembled the typical one for 50-year-olds would apparently be ageing 25 per cent faster than normal.

Already the molecular clock has yielded interesting findings. Men appear to age on average 4 per cent faster than women,which may largely explain why women’s life expectancy exceeds men’s by about 6 per cent. And the research has shed intriguing light on cancer: The clock indicated that tumour cells had aged,on average,40 per cent more than normal cells taken from the same patients.

If this continuing research pans out,ageing-rate tests may someday be standard in annual physicals,and tracking the results over time would offer unprecedented insights on health risks.