Horvath says that, remarkably, their analysis shows that some parts of the body age at different rates. When they used their algorithm on healthy breast tissue from two groups of women of average age 55 and 60, for example, it churned out a result that was on average two to three years older than the woman’s actual age. Whereas across both sexes, heart tissue appeared nine years younger than true age.

If it is known where the sample comes from, it is still possible to accurately predict age after some straightforward adjustment, says Horvath. However, in general, the algorithm is most accurate for samples from people under 30 years of age. “The older one gets, the less accurate it becomes,” he says.

Horvath thinks that breast tissue ages more quickly because of its constant exposure to hormones. Heart tissue may remain younger, by contrast, because it is constantly regenerated by stem cells.

Cancerous tissue also appeared to age prematurely, coming out at 36 years older than the person’s actual age on average across 20 cancers from 20 different organs.