This is a very mutlilayered question. As a biological species, humans aren't supposed to live as long as they do, so their sexual peak is at 18. It seems logical that, in some instances, that humans should be competing for the right to mate. However, whilst our bodies are supposed to be telling adholescents to be having sex beforehand, humanity has evolved into this mess we call society. Humans in society are tought that they should wait. People don't view teenage mothers as very responsible and they don't really command respect, putting it bluntly. They don't see these teenage parents as being experienced or mature enough to raise a child or, on the matter, have sex. This causes a conflict between biology and society. If biology is victorious, then the teenagers will have sex and raise their offspring. If society is victorious, the teenagers won't have sex. Of couse, there is always a stalemate, where the teenagers have sex yet don't raise any possible child, resorting to abortion or adoption.
Of course, both biology and society have slight quandries. In youth society, there is the matter of peer pressure. The age for kids to think it's OK to have sex is slowly lowering and, as mentioned, girls and guys as young as 13 are having sex. It might just be a myth, but supposedly, the world's youngest parents were 8 and 9. I question whether hormones are to blame for this. I'll be frank, I hadn't even noticed girls by that age. In this instance, I think that the peak of hormones is the best time to start having sex, at leat putting some logic into it and not making it just some school kid closet game.
Biology, however, does pose a taboo of society. In humanity's past, males have been challenging other males for the right to mate. Often, the larger, more experienced men would win. Society has now forbidden any such relationship. Society has made people wary of age differences in relationships, and has looked down upon those relationships with large age differences. My question is, would people prefer young teens having sex with older companions? If not, than hopefully teens having sex with other teens is the prefferable compromise.
Sex in a relationship is like the alcohol in a fine wine. Both are minor factors in a brilliant mix, but if you offer either to an adholescent, it will be these carnal elements that cause their wanting.
In my opinion, sex under 18 is OK. It could very wel be inevetible. However, to eradicate the barbaric, carnal element of sex would be to eradicate alcohol and other recereational drugs. Good luck with that.