Y'know, considering the artwork, and the way the PHB was constructed, it seemed designed to target the 15+ male adolecent.
I'm going to have to say, that I got into D&D, or atleast, fantasy novels as a Freshman in highschool. I wanted to play RPGs THEN, but didn't find anyone.
I will note that the first time I started playing 3e, and heck, I started DMing (And still have that game, I add), was when I was 17. I don't think the PHB should require you to be 18, c'mon. Sure, it will not cripple the hobby, it will not kill it, but D&D is not something that should be kept away from teenagers. I can get my hands on some rather violent video games, and I think D&D is even more harmless, content wise, then those.
It all depends on the DM. There are some DMs go game for their family, children and all. I imagine that they don't have these elements in their games. That's just fine. I imagine high schoolers experiment with the elements, but likely not very good. Morale delimmas are perhaps not seen with so much conviction and consideration as 'Let's blow it up'. I know some High Schoolers that would want DBZ D20. College students (When time is had), can incorperate Anthropology, Psychology, Geology, and so forth into their games, offering serious morale delimmas.
D&D is a System. A rules mechanic. So, WHY say 'you have to be 18 to use these rules'. Besides, if you really want to look at an RPG that targeted itself to kids, or atleast 'cutsefied' itself, then look at Changeling. Changeling had a Lot of potential, but they made it too cute. I quote a friend who won't touch it: "The first time we played, we went up against the Crayon Dragon." Usually, fairy tales are not very nice. Most were based on true, or rather grim things; look at the Brothers Grim. Now, how well did Changeling do?
But... I'm straying off topic.