18 is an arbitrary line in the sand...
You can try as hard as you want to stop teens from having sex, but it just won't work... they're not wired that way (from caveman days to the present). Teens screw around. That was true in your grandmother's day (as much as we'd like to romanticize the past and say it wasn't so), and it's true today. The best you can do is teach them the best you can to try and make good decisions and hope for the best. The stupid ones (and sometimes the just plain unlucky ones) will be saddled with responsibilities that they probably won't be able to handle (remarkably, some of them will handle it just fine, albeit with some bumps along the way that others won't have to deal with). I wouldn't want any young person to have to deal with that (especially my kids), but for a certain percentage of them it's inevitable. It's not "bad kids" either, it's pretty much all of them.
Abstinence has always been a battle doomed to fail. It's just contrary to human nature (like vegetarianism or pacifism). Encouraging your daughters to "save themselves until marriage" only made sense when you married your daughter off at 12 or 13.