Good Question..
...let her choose and don't "have" her go to either. Offer her the opportunity, encourage her, and if she chooses to go steer her towards the one she chooses. I have a meeting where, occasionally, a 13-year old from the Alateen meeting will join our meeting instead (she's been in Alateen since she was 10-- a remarkable girl, and mature in her recovery). I don't know what she gets out of it, but I get a lot out of listening to her. Another gal I've been in meetings with for five years was 19 when I first met her, and she was raised in an alcoholic home. I wish, at 24, I had the recovery she has.
There are no rules around this. Encourage her to try both on and see what fits.
Cyranaok
P.s. Gently, and this may not be true for you, many people I meet in Alanon including myself have children who are "younger" than their age because they were raised by codependent, enabling, and controlling parents like me. It's been very painful for my daughter to adjust to the new me, and to start acting like a 15-year old rather than a 10-year old. And it's my fault.
The reason I'm sharing this is if that's the case for your daughter, Alateen may be a better fit, but I still think she should choose for herself.