That biological clock is a kind of chemical change - methylation - that happens in DNA over time. Methylation is an epigenetic change, meaning that it does not alter the DNA code but rather influences whether genes will be switched on or off.
By analysing different patterns of methylation scientists can estimate a person's 'biological age'.
Four independent studies tracked the lives of 5,000 people for up to 14 years and compared biological age of blood samples with chronological age. The researchers found that a faster-running biological clock was linked with an earlier death.
The result held true even after adjusting for other factors like smoking, diabetes and cardiovascular disease. Even after accounting for these and other variables, people whose biological age was five years more than their actual age were 16 percent more likely to die during the period they were tracked.
Professor Ian Deary, of the University of Edinburgh, a senior author of the study, said the research team had 'identified a novel indicator of ageing, which improves the prediction of lifespan over and above the contribution of factors such as smoking, diabetes, and cardiovascular disease.'
Future research hopes to explain what is behind differences in methylation patterns.
'At present, it is not clear what lifestyle or genetic factors influence a person's biological age,' said Dr Riccardo Marioni, also from the University of Edinburgh.
'There is an ongoing project now to try to narrow down the effects of the precise relationship of biological clocks and mortality risks and what the other influences are,' he told the Daily Express.
The study was published in the journal Genome Biology.