First of all, "brain development" is not the only factor in deciding whether or not someone is ready to have sex. Sense and some degree of wisdom are also required. Decision making is much more about those things, and education, than about brain development. Humanity is disconnected from itself = paradox... But I'm assuming you mean the social stigmas and conventions are not reflective of natural human nature. In that case, what separates civilisation and wildness? My brain is developed enough to break a person's nose. So what?