A person's height usually reaches its maximum growth by fourteen years in girls and seventeen years in boys, and potential fertility and reproductive maturity usually reaches its peak one to two years beforehand in girls and three to four years beforehand in boys. All kinds of reproduction-assisting hormones begin flowing in the brain during this time, as well. From a strictly biological perspective, it appears evident that puberty is when we're supposed to begin getting busy.

Not that I'm for NVD, of course, but claiming that the body itself is unprepared to sex at that age seems pretty flimsy as a refutation.