Kayser hypothesized that sjTREC in blood could be an effective marker for age. He and his team developed a way to quantify the amount of sjTREC in blood, and analyzed 200 blood samples from people ranging from a few weeks old to 80 years old. They could reliably predict a person’s age, plus or minus nine years. Kayser took a second sample from volunteers and, after storing the samples for a year and a half, ran them through the same test, with similar results.

“The results were basically identical,” says Kayser. “We wanted to prove that if a blood sample lies around, that this time doesn’t influence the age of the person from the sample.”

The results are published this week in the journal Current Biology.

“The science behind this is innovative, and the test itself is a good addition to the forensic arsenal,” says George Sensabaugh, professor of biomedical and forensic sciences at the University of California at Berkeley. “However, the test likely would be useful only in a limited number of cases.”

Kayser and his team are looking for other signs of aging, both molecularly and genetically, to more specifically identify a person’s age.

“Estimates of age, of even 20-year increments, are valuable,” says Weedn. “With more technology, we may get the 20-year window to narrow. This is a start.”