I hate it when people claim that 13-14-15 year olds know nothing about sex. That's bullshit, I did it perfectly fine. So shit off.
I knew exactly what to do, what safety measures to take, what it would mean for the relationship, and how to ration it so it didn't control the relationship.
I did all of this at 14 years old, and I know for a fact, that most adults can't do any of that for shit.
Some adults are just as stupid as children in this respect, I find underage sex is ultimately fine. It is illegal to have sex 'underage,' but it cannot be enforced. Especially here in Britain, where the enforcing of this law is... well, non-existent. So it has to be regulated, which I am totally fine with. Condoms, and the pill are talked about extensively in schools, which is good stuff.
People are going to become sexually active at different times, and some people are going to engage in sex a tad younger than the law allows. At this point, you need to tell the young ones to take proper precautions, to stop the flow of babies. As long as there is no baby in the picture, underage sex is fine by me.
But I thoroughly hold the belief that nobody over 18, can have sex with anyone under 16. I think that's a bit off... And it should be off, because its pedophilia.