Blame it on factors such as smoking and stress, but according to a new study featured on TIME.com, we are older than we think.

Researchers at the University of North Carolina have come up with a way to test how old we really are based on our molecular and physiological age. They created a litmus test for a group of immune cells called T cells, and found that exposure to cigarette smoke and ultraviolet light aged mouse and human cells faster. Surprisingly, a high fat diet, which they expected to contribute to aging, didn’t appear to have any effect on the mice.

Despite these findings, researchers are aware that there are other factors that could be contributing to aging. And while there’s really no way to stop the aging process, we can at least slow it down by avoiding the sun, taking better care of our skin, and following a healthier diet.

(Photo by Caterina Appia via Flickr Creative Commons)