You can start the process when they are 3-4 weeks old. You make 2 little chick doors that only they can fit through. Then lock the adults out of the coop after they come off the roost in the morning and open the chick doors and bribe them out with meal worms or raisins or whatever they like near the door and out in the coop. Let them find the doors themselves. Then work on teaching them how to get back inside the brooder. Then let them explore the coop for an hour or so. Then open the pop door to the coop and let the bigs in and sit and monitor the interaction. Only intervene if a chick is in real trouble and needs help getting back to the brooder. More often than not, because the chicks have been raised in full view of the bigs, they accept them really quickly.