Sure there will always be some people who will do that. But isn’t having sex at 12 condemned in virtually all societies except for those societies (typically patriarchical and often polygamous) which marry off young girls at a very early age.
Yes, you're correct---societies, in an effort to modify normal human behavior, institute a variety of means of control, including laws and social disapproval. They wouldn't have to do that if human beings would just straighten up and behave "normally" would they?