Do your older hens free range at the moment?
I would allow the older hens out to free range and also the young birds. This way they mix initially when outdoors and able to get out of each others way. I would leave both run doors open so the youngsters continue to sleep in their own coop for another few weeks and also have their own food and hence no competition with older birds. In another few weeks the young birds will also be better able to defend themselves against established birds .... still quite young at 13 wks in my opinion.
A cockerel at 13 wks will not really be able to "boss" the older hens and establish himself as top of the pecking order. They do say not to put a cockerel in with older hens until he is mature himself as he maybe bullied and put off from doing his duty later on.
Not sure how true that is as mine always free range together first and are not confined in a run together until later.