If you consider the fact that most people hit puberty at age 11, 12, 13 (usually 12, it was 12 for me, at least) then you're asking these teens to go four or five years and be deprived of a basic human desire.
If two teenagers are at the maturity where they understand sex fully, and it's taught to them in our schools in such a way that they understand safe-sex and how to demonstrate safe-sex, then we can't really be the ones to tell people not to have sex. Not all sixteen year-olds are the same, some are mature, some aren't.
Hell, some 35 year-olds and even older than that are more immature than some of our 16 year-olds these days.
At this age they aren't adults just yet but they aren't children either they are capable of making their own decisions. We shouldn't try and stop it because it wasn't acceptable thirty years ago. I think that we should experience everything and at this tender age we do things that are either blessings or lessons. They're nearly classed as legal adults so what difference does a year make ?
There is so much emotion tied to having sex and kids under 17 are not mature enough to deal with it. Not to mention they are nowhere near ready to deal with any consequences, whether it be STD's or getting pregnant. I just wish telling kids under 17 not to have sex would be enough to stop it from happening. Our society is so sex based, we encourage teenagers who are just discovering their sexuality. Popular TV shows, movies, reality shows idolizing teenage mothers, even commercials are over run with sexual content.
Some cultures believe that children come to age at different times than our culture does, and some cultures believe that experimental sex is OK. Our culture tends to believe in an extended childhood and that sex should be delayed until one is considered an adult. I don't know that I would say that our culture has the better beliefs, but inside our culture young people having sex has a lot of bad consequences.
Although I think kids under 17 should not be allowed to have sex I do not believe it will stop them from doing it. Parents tell their children all the time to wait until they are old enough, but teenagers never listen. I think under aged sex distracts children from their school work and can lead to lower grades.