Wrinkle
A fold or crease in the skin that comes with age, sun and repeated expression.
Also: fine line
Wrinkles form as skin makes less collagen and elastin, the proteins that keep it firm and springy, and as it is creased by the same expressions over many years. Sun exposure speeds this up. Common places are the forehead, between the brows, around the eyes and around the mouth. Everyone gets them. Some traditions and portrait styles honour wrinkles as signs of age and experience; the cosmetics industry sells many products against them.