I have 4 Rhode Island Red hens at 8 months old. They are now laying and quite happy. I obtained 4 more chicks (hens) that are now 18 weeks and outgrowing their pen. At 16 weeks, I tried to introduce them to the existing flock, but the older hens just beat the crap out of them. I removed the youngsters for their safety. At what age should I integrate them into the existing flock? How can I prevent harm to the newer hens?