I had new chicks last spring in 2 batches 12 days apart. Everyone told me I could put them together, but I chose to keep them separate for about 10 days because the second group had a few banties who were too small to mix. Then I put them all into a large brooder separated into the 2 groups with a little fence between for a week. Finally I took the divider out and the groups could mix, but the older group kept the younger ones in the corner they had occupied, and did not allow them to mix. They are 22 and 23 weeks old now, and out in the pasture and in the coop they remain in their little groups to this day. A couple have started to roost in the other's area lately.