I too am having the same problem. My Buff Orphington hen's (2 of them) are about 9 months old.
My 4 new babies (various breeds.. Silver and Golden Wyondott's and 2 Rock speckles (?) have been next to the older gals for 3 weeks now. We let them mingle together in the run for a few hours and it was nothing but pandimoniam. Had to take the little girls out as I feared someone would get hurt. Lot's of chasing and pecking. I'll keep doing this but boy is it hard to see the fighting.
Sue
Sue, welcome to the forum, glad you joined.
In feet or meters, how much room do you have in your coop(s) and run(s)? How are they laid out? Photos could help. How old are your new babies, are they laying yet?
Until they mature chicks are lower in the pecking order than more mature birds. The more mature birds can be kind of brutal when enforcing those pecking order rights. That usually happens when the young birds invade the mature hen's personal space. They need enough room to get away from the older birds when they need to and enough room to avoid them in the first place. They should merge into one flock about the time the pullets start to lay but until then expect two separate flocks.