People are supposed to start having sex around 13. That being said, in our society, this can't really happen, mostly because of the way we've set up our culture. Someone mentioned earlier that "we should go back to the days when fathers sent their sons to brothels at age 13". Interesting and similar concept, but I don't agree with this. More important than anything is the creation of relationships. Humans use sex to strengthen those relationships. Sex shouldn't be used as it commonly is - as a tool of simple pleasure. But that's the outcome we've made for ourselves. That's what you get when you want everyone to be monogamous, and you put boundaries between age groups.