As with children, some pups develop at a faster pace than others. Train the fundamental element of each core skill progressively, and slowly compound success to form the learning chain. The fundamentals associated with an obedient, controllable retriever rest in four basic skills. These skills are the cornerstones for all future training. The skills are heel, sit, stay and here.
Age has less to do with it than temperament and maturity.
Teaching heel begins with tying and culminates months later with off lead heeling without any verbal commands. Good heel work requires polish throughout the dog's life just as all fundamental skills do. A dog should walk, swim and jog by your chosen leg, parallel at his shoulder-not ahead or behind. Your objective is that the dog will remain in a constant position on or off lead, sitting at your side, going over obstacles, running beside an ATV, walking through thick cover, swimming at heel as you cross deep water, etc. As a side note, never walk around obstacles or cover.