Researchers based in the Netherlands have reported a genetic signature for a person's age - to within a decade or so - in a type of white blood cell.
Manfred Kayser, a geneticist, and his team realized that the organ that pumps out T cells, the thymus, is gradually replaced with fat tissue as people age.
Every time a T cell matures in the thymus it rearranges its DNA to create a molecular receptor that can recognize pathogens and other foreign molecules, leaving loops of excised DNA behind.
Kayser and his team quantified the levels of one particular T-cell loop sequence in 195 Dutch volunteers, and plotted them against their biological ages, which ranged from a few weeks to 80 years old.
They found that they could estimate a person's age to within 9 years fairly accurately.
"The correlation is pretty impressive. How useful it will be in practice as a forensic tool remains to be seen, although there will certainly be forensic cases where it will help as an investigative tool," Nature quoted Mark Jobling, a geneticist at the University of Leicester, UK, as saying.
Kayser doesn't expect that simply identifying a suspect's age, but hopes the technique could be combined with other sources of evidence to rule suspects in or out.
The approach could also help investigators to identify victims from disasters, he said. Kayser also stressed that his team's technique would be used to identify suspects during investigations, not to convict them.
A potential limitation of the technique is that it could be a poor judge of age in people with HIV, diabetes and other conditions that perturb T cells.
The paper is published online in Current Biology.