I've got six chickens that are about 18 months old. Had them since chicks. This spring, we bought six more chicks and combined them when the the chicks were about three quarter sized adults. After three months, they still run around together in two separate flocks and the older chickens, one in particular, still peck and harass the younger birds. The chickens are all about the same size now, but the younger ones don't defend themselves. One funny thing is that in the younger set I have an Australorpe rooster (by accident). He STILL runs and cowers when the older hens come after him. Heck, I thought he'd be in charge, but he gets pecked just like the others. There is one clear leader out of the older group, and she takes after all the younger ones. The run size is about 6x25' so I'd think they'd have room. Will they ever integrate? Or does it even matter? I don't want the older ones interfering with the young ones when they start laying in a few weeks.