I've integrated chickens that age so it can be done. As was mentioned, get them used to each other first. There are a few other things that seem to help. Them having enough room is really important. The older ones will pick on the younger ones to set up the pecking order. If the younger ones have enough room to run away the pecking is usually not that bad. Occasionally you will have a hen that searches out the young ones to try to destroy them, but usually they just peck when they are in the vicinity. If you don't have a lot of room, make sure they have perches they can get to. When I lock mine in the coop and run, the ones lower in the pecking order spend a lot of time on the roosts, although they are a lot older than yours and have been well-integrated for months.
Try giving them different places to eat and drink. The older ones will often keep the younger ones away from the food and water. Some chickens have no problems sharing but many enforce their higher-in-the-pecking-order rights around the food and drink. If you can spread the food and drink out, it reduces conflict. Don't be surprised if the older chickens prefer the new feeding areas, even if it is the exact same food.
I find that bed time is another time that the pecking order is enforced. If you can, make sure they have plenty of roost space, maybe even as separate roost the young ones can go to.
I usually free range mine so my circumstances may be different than yours. I only intervene if there is blood or physical damage. In tighter space, I'd probably look more toward the relentless pecking also. The more room you have, the easier it is.
Good luck!