My 12-week olds are just about the same size as my older chickens. I decided to integrate the two groups today. They've been living side-by-side for about six weeks - with only wire separating the two pens. I've opened the door between the two pens many times - and they've gone in and out of each other's pens. Today, when all the 12-week olds were in the larger pen (while the older ones were outside free-ranging), I closed the door to the smaller pen so they couldn't get back in. I stayed to supervise them as they went to roost. There was some minor pecking as the older ones came in to roost. The younger ones were a little panicked not to be able to get into their own pen - and they didn't seem to know what to do. As it got dark, though, I picked them up one at a time and put them on the roosting posts next to the older ones. There was a minimum of squawking since it was almost completely dark by then. I've been out to check on them twice now - and so far so good. They're all settled in next to each other. I will head out early in the morning to make sure the olders ones behave. Wish me luck!