Welcome to the forums, Gruff, and I wish good hunting to you for a good gaming group.
With regards to age and maturity: responsible mature people in their teens are even rarer than responsible, mature 30-somethings; they're "platinum" to the 30-somethings' "gold", because these are the new wave of gamers that have come to the party recently, and they're the hope that the hobby will keep going. If I were putting together a new group, however, I would prefer 20 and 30-somethings to teens, mainly because I have more life experiences in common. That teen MIGHT be mature, married, and hold a job and pay rent like me, but it's more likely the older one is closer to my experiences and I'll be able to socialize more easily outside of the gaming table, which is important to me.
Would I accept a teen into my group? If he was obviously mature, then heck, yeah (assuming parents were comfortable with him/her doing so). Would I go around expecting to find a group like Nyaricus' group to join up with? Heck no, and further I'd have to be absolutely certain that parents, etc. were comfortable with it, because there are a lot of non-gaming issues to consider; I don't want even the slightest hint that I were doing something improper.
(Yeah, I know D&D is "improper" among some circles, but most rational people don't even think twice about it any more.)