With positive training, a dog can be started as early as 7 weeks of age. However, to teach formal obedience the dog should be older. 4-6 months of age is a good time to start formal obedience training using positive methods. One important note, in order for a dog to learn a new behavior they have to be old enough to be able to understand that behavior.
Teaching a 7-week old puppy to heel would not be a good behavior to teach simply because it takes concentration that a puppy of that age doesn’t have yet.