Peoples ability to think is not coded into the DNA.. Its has to do with life experience.
Biology and psychology say otherwise, that it is a mixture of both experience but also the power of the brain itself. That's why, with a poke or a prod or a zap of electricity to the brain, you can change some very fundamental aspects of a person's personality, to the point they may not even be considered the same person anymore.
Her body was ready and she gave birth right and she made milk and everything. so whatever.
The body is not "ready" until around 22-25 for women. While you can have a child before then, it is much like saying you can run a mile on a broken leg... sure, it's possible, but you are risking alot of long term health issues and a load of unnecessary pains. Simply put the skeletal structure of the female body is not in it's ideal shape, nor is the size or ability to produce nutrients.
Pregnancy takes ALOT of energy, as does puberty and growth during the teens. If you cant produce enough energy, it doesn't magically come from no where; the body takes it away from either the growth or the pregnancy. And unless you are eating steak after steak after vegetable after vegetable after (etc. etc. ...) all day long your body will NOT be getting the amount of nutrients it needs to provide for either you or your baby properly. That is an unnecessary health risk for both mother and child.
Then lets say she breast feeds (something that should be HIGHLY encouraged for any potential mother because of all the health benefits for not just the baby but the mother as well)... again, this requires ALOT of nutrients, and if she cannot keep up with it, either the milk or her body will not be receiving all the nutrients it requires.
Whatever.. she wanted to have a kid so she had one "free will" and all that jazz.
I want a boat... therefor, if I go and steal a boat, does that mean it's okay because I "wanted" it?
But that doesn't really do any physical harm, so lets change it...
I want to go stab someone... I really like playing with knives and think it would be pretty cool to shank someone... so does that mean it's "okay" because it was my free will to hurt someone else?
No, it doesn't. And when you have a baby before your body is ready, that is what you are doing; intentionally hurting someone else.