After they have all thier puppy booster shots, which is about 12-16wks.
I took my dogs to PetSmart and was very happy with it. They have since changed some of the things they do but still over all a great program.
But first go in and find out what time the classes are so you can observe the trainer and get a feel for themand also talk to them. I talked to my trainer before putting my dogs in and was very happy with her. Reggie now has his CGC and my other dog I had at the time went through all thier classes but wasn't able to get her CGC because the AKC changed thier standards about the instructors giving the test.
I don't know if your Petsmart has it, but the one I went to also had puppy play groups where you were able to take your puppy in and let them play with the other pups. But you either had to be evalutated by the instructor or have attented a class to participate.