We started at Petsmart when Stella was 5 months; she graduates in two weeks! She's done really well with the clicker training, but she does get really distracted by the other dogs and acts crazy during class (even though she's usually super calm at home). The trainer thought it was because the rest of her class was little boys but it didn't change much after she was spayed. Regardless, she's learning, but I've been working on it a lot with her at home. I think that's the important thing anyway. Before we started classes, I had already taught her to sit, come, and lay down without a clicker--I just read some internet articles on the best way to do it positively, it's not that difficult once you understand how to go about teaching behaviors.
I would wait a couple months, but that doesn't mean you can't give her a good head start before you start class
