Most men's hairlines move back modestly from their teenage position — a centimeter or so — and then stabilize. That is a mature hairline, not balding. A receding hairline keeps moving, often faster at the temples, and is usually accompanied by gradual thinning behind it.
Transcript
Most men's hairlines move back a little between the late teens and the late twenties. That's a maturing hairline, and it isn't loss. The difference is the shape. A maturing hairline sits slightly higher and stays even. A receding one goes at the corners first, so the middle stays forward while the sides retreat, and it keeps going. Even is usually fine. Uneven is worth a look.
You cannot tell the difference from a single look in the mirror; you tell it from change over time. Two standardized photos taken months apart answer the question definitively, where memory and bathroom lighting cannot. A doctor settles it fastest: with Uberhair's free hair check, a network doctor gives you an initial assessment from photos you take at home, and network clinics then measure change over time with GroTrack imaging.