I think it depends on the dog as far as when to start. You just have to make that call your self. Some dogs do good under a year old. I've posted this pic before and the black and tan dog is 8 or 9 months old in this pic and hung with the two other dogs for two miles before I got there. And she's never been in a bay pen. Just the woods, maby that had something to do with it.
Here she is last sunday.