American researchers said on Thursday they can use saliva samples to predict a person's age, a discovery that could help CSI investigators track suspects and open the door to personalised medicine.

Using a process call methylation, a chemical modification of one of the four building blocks that compose human DNA, scientists at the University of California at Los Angeles (UCLA), were able to determine a person's age to within five years.

"With just a saliva sample, we can accurately predict a person's age without knowing anything else about them," said Dr. Eric Vilain, the director of the Centre for Society and Genetics at UCLA.

He added that if some saliva is found around the butt of a cigarette, scientists could use the technique to tell if the saliva is from a person in their 20s or their 70s.

"It could also be used for victim identification in which you have very little and have no idea who the person is, whether it's a young or an old person," Vilain explained.

Vilain and his team, whose findings are reported in the online edition of the Public Library of Science One journal, made the discovery while studying 34 pairs of identical male twins with different sexual orientations.

They studied the men's genome and found sites on DNA that linked methylation to age.
After identifying two or three genes with the strongest age-link to methylation, they used data from the twins and 60 men and women to predict their age.

Vilain said the findings could potentially be used to determine how a person is ageing biologically versus their chronological age, or how different environments age a person.

"This opens a door to truly personalize medicine based on one's physical characteristics, rather than on age alone. For example, it's recommended that everyone should get screened for colon cancer at age 50," Vilain said. "Some people could wait longer, or need it sooner because their body in general ages faster."

In the long-term, Vilain hopes the discovery could be used to determine ways to slow the ageing process.

"I'm dreaming here, but if ageing is at least partly influenced by the environment, then one could wonder whether you could change the environment," he explained.

"I'm really interested to see how you could slow the process down, maybe by giving supplements, changing the environment, going different places ... it's a dynamic process," he said.