Why do you suppose teenagers can have children?

Yes, I know biologically why they can.. but from a I guess "philosophical" standpoint, I wonder why. Teenagers are obviously not grown up mentally enough yet to be considered adults, yet mother nature allows for them to take on such an adult role of motherhood if they choose to. It almost seems like a contradiction in nature in a way. I know this is an odd question that probably nobody will answer but for some reason these thoughts just came to me the other day..