It's an automatic behavior when you meet someone new: subconsciously, you take stock of physical cues to gauge how old they are. Facial wrinkles, gray hairs, an unsteady walk: these all signal older age. Often, these guesses are pretty accurate, and researchers are attempting to replicate that internal age calculator to figure out how old people are—not chronologically, but biologically. In a new paper published in Nature Medicine , Tony Wyss-Coray, professor of neurology at Stanford University