It may sound stupid, but if you are not allowed to roleplay up to the age of 18 you won't instantly be a genius at roleplaying once you reach that age either. It is pretty clear to me that the increased ammount of bad roleplay under younger players is not caused by the fact that they are younger, but by the fact that they are more likely to be unexperienced. Any unexperienced player will make mistakes, and sometimes make himself look like a bad roleplayer, no matter whether he is eight, 18 or 75. When I started playing Planeshift I might as well have been "to young" for it, and to a degree I still am, and I am pretty aware that my RP was abbysmal, in the beginning, being mostly dominated by twisted images of my famous roleplay characters, at that point. Nowadays I consider myself a decent enough RPer, but I can only speak for myself, when saying that this was not caused by the sudden enlightement of coming off age, but rather by a lot of trial and error and quite an ammount of hard earned experience. I am pretty sure that my roleplaying would not be any better at all at this moment than when I started playing PS, if I would not have earned this experience. Of course, you might say that Planeshift should be a place only for those who have already got through their first years of RPing, but I do think it might not appeal to fully fledged profi-RPers who already found their place in the vast world of roleplaying.
Edit: Typos :X