June 22, 2011 -- If you want to hide your age, don't spit.

UCLA researchers' new saliva test can tell a person's age within five years -- so far, the most accurate test for age.

"With just a saliva sample, we can accurately predict a person's age without knowing anything else about them," Eric Vilain, MD, professor of human genetics, pediatrics, and urology at UCLA's David Geffen School of Medicine, says in a news release.

The test looks for a kind of DNA decay called methylation in just two sites on the genome. These two sites are among 88 sites on 80 genes for which methylation changes are particularly well linked to a person's age.

Vilain and colleagues found the sites during an analysis of 34 pairs of male identical twins between 21 and 55 years of age. They then tested the saliva of 31 men and 29 women aged 18 to 70.

The test was accurate within 5.2 years.

The test explained only 73% of the variance in age. In other words, the test said some people were younger or older than their actual age.

Vilain suggests that the test correctly identifies a person's biological age -- bio-age, as he puts it -- which may be more relevant to a person's health than the number of birthdays a person has had.

"Doctors could predict your medical risk for a particular disease and customize treatment based on your DNA's true biological age, as opposed to how old you are," he says.

Vilain's team is looking at whether people with a relatively young bio-age live longer and healthier lives than same-aged people with older bio-ages.

Vilain and colleagues report their findings in the June 22 advance online edition of PLoS One.