That behavior sounds pretty normal. More mature chickens outrank less mature in the pecking order until the younger mature enough to join the flock as equals. Especially if the less mature invade their persona space they aren't shy about enforcing their pecking order rights. My pullets tend to meet that maturity point about the time they start to lay.
I have a lot of room outside, even when it is really crowded they have over 60 square feet per chicken. My younger ones form a sub-flock and usually stay well away from the adults. Each flock is different and sometimes they do mingle some, usually when a brood is being raised by a broody hen, but in general they stay well separated even then. When I open the pop door to let them out in the morning it is pretty normal for the younger ones to be on the roosts while the adults are on the coop floor. They are avoiding the adults and probably for a good reason.
My definition of a successful integration is that no one gets hurt. All that stuff about them hanging together during the day and sleeping together at night will eventually come but they usually need to mature enough to do that.
The more you can tell us about what you have to work with the more likely we can give suggestions that suit your circumstances. That includes the size of your coop, size of the run, and how you manage them (when that run space s available). A general location so we know your weather can often help. Photos can be very valuable so we better understand what you have to work with. An example, my roosts are 5 feet high. The juveniles can go up there to find a safe place. If your roosts are low enough that the adults can peck the juveniles on them the roosts are not a safe place.
What you are seeing is not that unusual, many of us go through it regularly.