I put some ~5 week olds in with ~7 week olds. It was rough on the babies.
I took the older ones out except for one.
I put a relatively gentle pullet from the older group in with the babies for about 4 days so that they bonded somewhat. That seemed to help alleviate a lot of the segregation, and the pecking order seemed to settle out better when I re-mixed them. Also the 5 week olds gained some size during that time too.