Did you know that babies aren't born with knee caps?
Babies start to form cartilagenous material on their knees in utero - but that material doesn't ossify until a kid is 4 or 5 years old.
Some people think it has to do with evolution. That is, not much evolutionary benefit to having a kneed cap until one is walking.
The same thing happens with the pelvis, it isn't fully ossified in women till they reach about 18 years. No reason to spend the extra energy resources to ossify that part of the skeleton until to ability to bear the weight of pregnancy is possible.
