Age doesn't matter, as long as you follow the "see don't touch" method, which you seem to have started with, so that's great. They need to be behind a divider where the flock can see them, but not get at them, for several weeks. Then start letting them mingle with the flock, for short supervised periods at first, and increase their length every day. So if you have half an hour to spare, start with half an hour of them loose with the big girls, with you hanging around supervising. Then put them back. If that goes well, you can start increasing the time the next day, and maybe you aren't in with them the whole time, but nearby (doing yard work etc.). And so on. Eventually they'll be able to survive a whole day with the flock without your supervision.Since you're experienced with introducing young ones... hoing you can make a suggestion of at what age to introduce and at what level of oversight? I've read to be there all day to mediate, or just to add at night and then have a space with food and water that only the young ones can enter. have 3 six-week old hens that went to sleep tonight in the big girl house on the roost along with the other 8 one-year year old hens. Wondering if I need to move them back to the young girl house since I won't be there to protect them in the am? At what age can you mix? They were in a kennel in the big girl house for 5 weeks, then in their own condo but within sight. Any suggestions appreciated. Thanks!
Nighttime is trickier though, because who sleeps where is a big deal in the chicken world. That's when the pecking order gets enforced, and lower ranking chickens tend to get pushed around. It's also a tighter space inside the coop, and more likely for the newcomers to get within the elders' personal space and to piss them off. So I would wait until they can spend the day together without incidents, before you let them sleep together at night. And try to hang around nearby for their first bedtime together, just to keep an eye on things, until they settle for the night. The new ones may need help/encouragement from you, to go inside the coop for the first few nights, because they'll be apprehensive of going into an enclosed space with the elders, who'll boss them around in there for sure. This is also why I don't like the "throw them in there after dark and they'll be fine" method (without see-don't-touch first). That's a recipe for disaster. Chickens aren't stupid, and will see the intruders first thing in the morning, in a tight space with no escape (if the door hasn't opened yet).
How successful an introduction is and how long it takes also depends on the temperaments of everybody involved. Some breeds/individuals are more prone to conflict than others. So take it slow and see how it goes. And remember that there will always be some pecking order enforcements, even years after the introduction, and the different generations of chickens will probably never be "friends". But the goal is for them to tolerate each other without plucked feathers or blood. If you can get that, then the introduction is a success.