How old are you? Spit it out! A new UCLA study shows that your age can accurately be predicted from, of all things, your saliva.

It's not as ridiculous a scientific endeavor as you might initially guess. The researchers, led by principal investigator Dr. Eric Vilain, explain that the finding could serve as a forensic tool for crime scene investigators to accurately pinpoint the age of a suspect, narrowing the age of a suspect to a five-year range.

"Regular DNA analysis at a crime scene gives information on the characteristics of the person that are immutable," explains Vilain, a professor of human genetics, pediatrics and urology at the David Geffen School of Medicine at UCLA. The "immutable" characteristics he lists are things like a person's DNA sequence and how it matches with suspects. But Vilain explains that the "spit test" could give investigators valuable information on a characteristic that changes with time, which "could help the investigators zoom in on the right people."

To find saliva's age-predicting power, Vilain and his team of UCLA geneticists used a process called methylation, a chemical modification of the DNA, which is influenced by our environment -- the food we eat, the toxins we're exposed to, "and, as it turns out, time passing," Vilain says. 

"When methylation happens at certain places in the DNA, it tells certain genes to turn on or turn off. So the sequence of the genes themselves is not modified, but their expression is," he explains. "What we found is that the degree of methylation at a small number of places in the human genome is linked to our age. The correlation is high enough that we can predict what the age of a person is by just having access to a sample of their saliva."

The weirdest part about this very weird research: "It was an accidental finding," admits Vilain. He and a team of researchers rounded up 34 pairs of identical, male twins, ages 21 to 55, for a study on differences in methylation when it comes to predicting a person's sexual orientation. That study was a bust, from a scientific point of view -- but it did lead them to finding the spit test for age. They used saliva samples to scour the twins' genomes, and identified 88 sites on their DNA that correlated methylation to age. (They repeated the findings in non-twins -- 31 men and 29 women ages 18 to 70.)

They then identified the top two genes with the genes that were most highly correlated to age, and used those to build a predictive model that could correctly predict the person's age within five years.

We're talking about a very small sample of saliva -- about 0.1 ounces, Vilain says. Thanks to this unexpected finding, he's now surprised to be asking himself research questions inspired by an episode of "CSI": "Can we do it with saliva on a cigarette butt?"

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