I am going to assume you mean you and your dog attend obedience school together. I do agree. Never send your dog away to an obedience school. But that would be another question to respond to. :)
Once your pup has had it's three rounds of shots, it has the antibodies to attend puppy classes. My pups start training at home the moment I get them. However, even though I'm a professional dog trainer and can obviously train my dogs not only the basics but highly advanced skills (in fact, really anything I wanted to train I could), puppy class is important for the socialization. (I have a hub on the importance of socialization and how to do it properly). Taking a puppy to puppy class allows it to be around other pups, other people, a strange environment and yet be asked to focus and work amongst all of those distractions. I prefer and seek out puppy classes with puppy play time, as long as the instructor is good. Unfortunately, some of the big pet mart stores do not hire experienced dog trainers, but rather just "cashier" out of their staff. These people don't have the years of experience to really help owners find various ways to reach their individual dogs.
Although I start my pups in classes (for the socialization, which is a HUGE deal) as soon as they have their antibodies from their three shots, you can put a dog into obedience at ANY age. I have students who come to me for agility training with 9 year old dogs, and still they get into competition and earn high level titles with these old kiddos. As long as the dog still has its mental abilities, it can learn, regardless of the age. :)