Ephebophile.
Look. The human body is messy. It's an amazing, intricate machine, but it's messy.
Here, let me tell you a story.
Once upon a time, some primates underwent sexual selection not unlike that which resulted in the male peacock's tail. Turns out they liked music, dance, and art. It also turns out that those with an above-average brain size were better at making music, dance, and art then other members of this specific species of primate.
So this went on for a very, very long time. Eventually, the descendants of those primates had brains large enough that the head and skull had to grow to cover them. This led to an issue for females giving birth. The ones with wider hips were more likely to survive squeezing out the massive head of a baby, so over the generations, wide hips in females were selected for.
It was basically an arms race between brain and hip size until babies just started being born earlier and earlier because that worked better than making the hips even wider.
The current form of that species is Homo sapiens sapiens.
So we've established that wide hips are needed to give birth in humans. (Look at purebred pugs to see the result of large heads and small hips - they can't give birth without a C-section.)
Now look at the average hip size of a grown woman.
Then look at the average hip size of a fourteen-year-old girl.
"But she's fertile! She has her period!" And this is where the top of this wall of text comes in handy. Evolution is messy. The hip fiasco didn't sync up with the age of fertility. Evolution responds to pressures from the environment, not to some directive leading to a goal, and there was no substantial pressure pushing female fertility later. (Edit: most probably because most men do not want to bang fourteen-year-olds. Not enough pressure to push reproductive age later/hip growth earlier. So things stayed more or less how they were.)
The end result is that it is very possible for a girl to get pregnant before her body is capable of carrying and birthing the baby. This usually doesn't end happily.
