The question is: why do teenage boys look like monkeys?

The human species is highly neotenic. That is, a fully mature human is actually still immature compared to what the genes could do -- an adult person is much like a chimp child: hairless, playful. Like dogs are neotonic too: a fully grown modern dog has unfolded its genes as much only as a puppy wolf.

This implies that during some period of humanity's evolution, the species' neoteny increased from nothing to where it is now. Say for example it took a thousand years, it doesn't matter what the evolutionary pressure was. During that thousand years, the mature adult of each generation would look more like a younger chimp than the mature adult of the generation before.

Now. Consider that, to reproduce, men both young and old would prefer to mate with young women: Women bear children best when they're younger, are less likely to die and more likely to nourish the child properly, etc (the grandparents are more likely to be alive when the mother is young).

Whereas women will prefer to go for older men: Having a child is a large investment for a woman, and an older man has proven survival skills: The fact that he's actually got older means he's intelligent, strong, canny and so on.

So the older man has no trouble, but it's in the young man's interest to trick the woman. He can't just evolve to look older when younger -- or rather he can, but this isn't an evolutionarily stable state, because old men and young men will get in an arms race to look older, which the old man will always win, and so the race peters out.

However, the young man can take advantage of the fact that older men are also of the generation before, who are less neotenic and look more like apes. This won't actually mean the young man will genuinely become more ape-like - this would leave him vulnerable to the evolutionary pressure which is promoting neoteny to begin with - but it means he will attempt to appear more like an ape.

The old man - who is old and apish - and the young man - who simply appears apish - can now compete on equal ground to mate. As the young man gets older, he competes with those of his own generation, so the monkey-characteristics recede. Although the constant increase of neoteny has now ceased, we're left with its effects.

That's why teenage boys look like monkeys. And what of the young woman? Having lost an important discriminator between old and young men, she moves on to secondary characteristics of age, such as: does he have a car, is he old enough to get served in pubs.