A stranger who wants to know another person's age can either ask for identification or look at the person's face, and the latter is less intrusive and almost as accurate. People are able to determine age from the face with surprising accuracy, to within a few years when the person judged is between 20 and 60, a skill that develops early in life when even babies can distinguish children from adults.
aging is when you stop growing new cells as fast as when you were young, and your current tissue starts to deteriorate.