I brood mine outside and let them roam at about 4 weeks or so. Sometimes I tractor them for 2 months. But I don't force them to sleep with the big girls till about 4 months old despite ranging together for a few months by then. If it is just one, wait till they are about the same size, or the big ones can really do some damage to the young one with picking and their panicking to get away. Of course, if you don't have multiple tractors and just a few birds, you can pop them together earlier and probably get away with it. 20+ adults vs 1 baby usually doesn't turn out well.