I find the best age to integrate is 2-3 wks....they aren't so small they can't escape the big birds and they aren't so big the big birds see them as food competition. Especially if you have a free range setup, as the young chicks hang near the coop and the older birds are outdoors most of the day. I've never had a chick injured by integrating at this age and never even saw any overt aggression towards them at that age and they all eat out of the same trough together.