The younger group is more skiddish so I’m afraid if things don’t go well it will be difficult to get them back to their area.
Keep the young ones locked in there until they are trained to put themselves to bed in there at bedtime. That can vary by chicken however, no set time. I'd give them at least a week, longer might be better. This has nothing to do with getting them back in there during daytime. It only works when their instincts tell them it is time to go to bed.
How long before I can integrate the 2 groups?
Do you have a run outside? That can make this a lot easier. The more room you can give them the better.
My goal when integrating is that no one gets hurt. If they gather in a cuddle puddle, sleep together, or eat out of the same bowls at the same time fine, but I'm equally happy if they form two separate flocks and keep their distance. All that cuddle puddle stuff will take care of itself over time as long as no one gets hurt.
By the time the young ones have learned where to sleep, the two groups should have gotten used to each other. If you have an outside run I'd open up access to make that additional space available. Then open the door between the two groups and see what happens. They may merge into one group peacefully. They may form two different sub-flocks with the younger avoiding the older. The more room they have for this the better. This is what I'd expect to happen most of the time, two sub-flocks. The danger is if the older attack the younger, especially if they hold them down and start trying to peck a hole in their head. If this happens, separate them immediately. They can kill doing this so pay attention. If you see this isolate them and come back on here with a description of what happened so we can discuss what to do next.
Your amount of room in there isn't bad but it isn't great. You do have more room than many people that integrate. The young ones will probably need room to avoid the older. You can improve the quality of what room you have by adding what we call clutter. That is adding stuff they can hide under or behind to break the line of sight. They are old enough that they can fly up on roosts also. Others have already made suggestions on how to do this.
Having multiple widely separated food and water stations helps a lot too, especially if they are out of line-of-sight of each other. That way the older ones cannot bully the younger away from the food and water.
I suggest you do not force them to stay in a small area right next to each other, day or night. They are living animals so I cannot give you any guarantees as to what will happen, but with mine they usually need enough room to avoid the older. Do not believe they immediately need to sleep on the roosts together, eat or drink together, or hang right next to each other. All of that stuff will come later as the young mature. Your immediate goal is that they do not get hurt getting to that phase.
People do this all of the time. Sometimes it is pretty easy, sometimes it is really stressful. It usually works. Good luck!
