Want to know how old a tree is?

Spit in your hands, pick up a saw and slice yourself a nice cross-section of hardwood.

Trees produce new layers of wood around the trunk, just under the bark, that correspond to growing seasons. Count the layers and you have a general idea of how old the tree is.

But that’s not a good approach to figure out how old your blind date is.

There are no rings to count, and it would get messy.

But scientists are working on ways to calculate someone’s age … no bothersome chopping or sawing required.

Like the tree technique, the first step involves spitting.

By searching saliva for age-related biomarkers, UCLA geneticists say that can very determine someone’s age within a five-year window.

The technology is new and in early development.

No one expects to see it being used on date nights in the foreseeable future.

But forensic experts may soon be able to analyze traces of saliva left in teeth marks or in drinking cups to determine the age of the age of a criminal suspect.

Technically, the process involves extracting DNA out of the saliva and examining a chemical process called methylation.

Methylation occurs every time our DNA replicates itself, and it can be tracked very much like growth cycles in a tree.

But there is a hitch.

We all know people who appear much younger or older than they really are. In a minority of folks, methylation rates and chronological age do not match.

When that happens, UCLA researchers believe the “methylation” age corresponds to a person’s true “biological” age.

In that case, the age-predicting technique may have an important medical use.

Certain health screenings, such as colonoscopies or mammograms, correspond with age milestones. Researchers say a person’s bio-age might provide a better benchmark for timing disease prevention efforts.

Now, that kind of information doesn’t grow on trees.