It's one of the weirdest changes to happen to a man's body throughout the history of civilization. There's one thing to be said about evolution but this has scientists everywhere mystified. It used to be that only a certain amount of men would have to contend with male pattern baldness and it was largely genetic whether or not you got it. But over time, the so-called "male pattern baldness" genes became more and more common to the point where every single man on the planet goes bald and it happens now during puberty.
Every guy will have a full head of hair during childhood. Then at around 11 or 12 years of age the receding hairline starts, the thinning of hair in the crown starts at around 13 or 14. At about 17 years of age, there will be no hair left on the top of the head and all that's left is the rim of hair around the sides and back of the head.
Some guys have no problem being bald for their entire adult life (it turns out that a lot of chicks do dig the bald look) but not everyone does. That's why scientists have developed a drug that could potentially reverse the hair loss process completely over the course of a few years, but it's very experimental and could have some strange side effects.