Did you get the 8mo pullets and roosters recently?
Can you divide up the run, or make a penned area for one group?
I have just introduced three pullets (about 8mo) to three older ones. I got pullets this time because the existing girls are too mean on a newbie that they saw growing up (even penned separately to 16 weeks).
What happened this time, the older ones were quite intimidated by the pullets, not realising that their big bodies housed a 'teenaged brain'. As my alpha (9yo, which is what I call an older bird) put herself out the mix first by spraining her toe joint, then breaking her toe due to the sprain.
I had the run divided into half, and the pullets slept in a caged off area in the coop. At 8 days I did some part time introduction between the two groups in the free range area, and told each pullet 'no' each time they tried to bluff the hens up the pecking order. That only happened for a couple of days, and nothing too major.
Older and newer birds now free ranged or in the run without issues. I still have not integrated them in the coop yet, due to the 9yo being in the dog cage and taking up too much extra space in the roosting area. But I will be integrating them in the coop this week. Mine are pets, so I closely monitor the behaviour of each individual.
I have never had roosters, and having even young roosters changes the dynamic dramatically. From the behaviour of your older hens, I would say they are intimidated, not cold (chickens don't mind the cold).