Honestly, I've never seen an 18+ Roleplay that has actually been too mature for a high school student.
I temper this with several caveats:
-I'm not saying that the 18+ restriction doesn't exist for some legal reasons (as weak as they are). It does. They should be respected for that, for reasons stated above.
-I'm talking about 18+ roleplays that don't actually venture into X-rated territory. Graphic sex, abuse, torture, rape, I'm sure those exist in roleplays somewhere. On the whole, R-rated/18+ roleplays don't usually involve those subjects, and I am speaking about those roleplays in particular.
Now, back to what I said. Most high school students, especially upperclassmen, are basically mature enough to handle heavy violence, sexual drama, romance, and most of the other material that comes up in 18+ roleplays. Plenty of them watch or experience it already in movies, video games and on TV. There's also no shortage of places on the Internet where you can find that kind of thing. There are a few exceptions, some high schoolers are simply not mature enough for this, but so are there exceptions for actual adults who can't handle that kind of thing.
So the question about maturity is usually pretty null. Roleplayers tend to be fairly young, anyway, from high school to college aged, with a few outliers on either ends. There are plenty of college-aged folks who are more immature than some high school freshmen, and since everyone really matures at a different rate, it's hard to pinpoint an actual age. 18 is pretty arbitrary when you think about it, but it's usually chosen as it coincides with the age of legal consent.
And that's the key here. Consent. Legally, a minor cannot consent to a whole host of actions which s/he would otherwise be mature enough to handle. Not because there's some conspiracy against them, but because the law was designed that way. For some, it sucks. For most, it's just right. For a few, it's not enough.
Could you join an 18+ roleplay, act mature enough and avoid detection? Yep, probably.
Should you? No. If, and it's a big if but the if still exists, the Guild was ever discovered to have minors participating in mature situations, Mahz and the Guild would probably come under legal suspicion. It can cause problems. RPG is probably not as big of a target as the old Neopets once was, or other big boards like that, but the threat still exists. The best way to avoid this and keep the Guild away from that kind of trouble is to respect the 18+ restriction and wait a few years.
Here's an alternative: Short stories that you write yourself don't have an age restriction. If someone really loves writing mature scenarios, but they're not old enough to join a roleplay, writing short stories on their own can be a creative outlet to keep the momentum going. That one's between you, your conscience, and your deity. Just don't share them until you are 18.