My flock is generally laid back. I have never had a hen that aggressively goes after younger chickens. If a younger chicken wanders into their territory, they will peck them to remind them that inferiors don’t invade the private space of their social betters, but when the younger one runs away, it’s ended. The hen does not chase them to cause harm.
I house my chicks in the brooder in the coop from Day 1. They can see each other. When they are around 4 to 5 weeks old, I move them to a grow-out coop and run, but again they can see each other. When I am comfortable the chicks have learned that the grow-out coop is their home and they should return there to sleep at night, I just let them out to free range with the flock. Usually this is somewhere around 8 weeks but I have done this as early as 6 weeks.
What normally happens is that the younger ones get pecked when they try to mingle with the older ones. They quickly learn to stay away from the older ones. I have separate feeders and waterers set up so that is not a conflict point. The younger ones find their own place to hang in the shade during the heat of the day and their own place to dust bathe. They go to separate sleeping quarters at night. I find the worst bullying takes place in the coop as they are getting ready to go to bed.
I’ve had a broody wean her chicks as early as 3 weeks. Those chicks were raised with the flock and slept in the coop with the adults. I set up a separate roost, lower down and away from the regular roosts, so they could get some separation from the main flock. Those chicks were picked on if they tried to mingle with the main flock, so they stayed separate. If I was a little late opening the coop, those young chicks were either on the roosts while the adults were on the floor, or they hid under the nest boxes where the adults could not get to them easily.
I really think room is the key. If space is tight to where the younger ones can’t maintain that separation, you are more likely to have problems. But if you can give them extra perches or roosts and places to hide under or behind to keep them out of the adult’s sight, it will be easier.
But occasionally you get a hen that is especially aggressive. They can make it a lot harder. I wish you luck.