Hi Jezzeb! Gee, I wish I had gotten a handle on my eating issues when I was 15. Good for you!
The teen thing: it depends on your area. Some places (big cities) have groups that are for younger members. In a typical, non-speciality group, you'll find people of every age. My thursday night group's youngest member is 19 (she's away at college right now, but comes back to visit when she can). Our oldest member is...mmm...60s, I'd say. And there's only about 15 of us, so it's quite a spread agewise.
You don't have to sign anything or join or be official. You just keep coming to meetings. There are no dues or fees or anything. You don't even give your whole name, you just say "I'm < your name > and I'm a < compulsive overeater, bulimic, anorexic, addict, new, just visiting, whatever >" when you're doing introductions at the beginning of the meeting. No one will question or challenge you. You don't even have to say your real name. No one will know.
You don't have to be hardcore ANYTHING to be part of OA. I'm not a Christian, and I'm a part of it. Your Higher Power (HP) is your own business, and people won't challenge you on that. The program is based off the idea that you need a closer relationship with your own HP, and it's built around helping you get that. When I came into the program I had NO relationship with a Power of any kind, higher or otherwise. What I did was make up my own. I wrote a want-ad for what my ideal HP would be, picked out a name (the Welsh word for 'hippopotamus'), and acted as if that was the right thing. And it was. I don't 'pray' to my HP: we chat. If I listen closely my HP will say "that's enough" when I'm eating, and I'll know to stop. I can turn over anything to my HP, because if it's bugging me then she (yes, it's a she) wants to help. You do what works for you.
And it's not like a church meeting. There is the serenity prayer at the beginning ("God grant me the serenity to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference") and individuals may mention their HP, but NO ONE will comment on anything you say, or challenge you with gospel or whatever. There is generally a closing prayer, but you don't have to say it. It's been 7 months and I still don't know the prayers.
I have no intention of ever learning them, either. They're other people's words, and they don't work for me. Take what works for you, and leave the rest.
Give it a shot, see what happens!