How old are you really? Chronological age is easy — count forward from birth. But establishing physiological age is purely subjective, based on how someone looks, feels and functions. Research in nematode worms is providing a crucial first step toward the development of biomarkers to predict chronological and physiological age in humans. This technology could facilitate testing of anti-aging therapies as well as give physicians a means of tracking how individuals “withstand the tests of time.”