if you don't want anyone knowing how old you really are -- watch where you spit!

UCLA scientists have figured out how to tell a person's age by analyzing saliva samples.

They scoured the genomes of identical twins, finding DNA sites that played a role in aging.

They were able to predict a person's age within 5 years. They say the test could be used someday as forensic tool -- getting information about a person using saliva traces left on a coffee cup, for example.