The age disparity makes co-housing them problematic for a few reasons- size/aggression, environmental needs (your older bird-s have likely nearly fully feathered out, the younger birds are still in need of supplemental heat, etc). As you have seen, putting them together is a no-go at this point and will have to wait until all the birds have reached a point of being able to hold their own and deal with a merging of their groups.
