It's really very easy to teach youngsters to roost on a perch. You place them up there, and keep putting them back when they hop down.
There are two secrets to making this work. One is to do it as it's getting too dark for them to see. They'll want to be settled as it gets dark because they lack confidence and don't want to end up all alone in the dark.
The second is to place them very close together so they're touching. This is why they prefer to puppy-pile in a corner on the floor, for the comfort and reassurance they get from each other. When I first place them on the perch, they're nervous and a bit afraid. I place my hand gently on their backs until they calm down and settle on the perch.
Just last week I moved my six five-week olds into the coop. It took three days and they were hopping up on the perch all by themselves. A week later, they are putting themselves into the coop, as well as navigating the plastic flaps over the pop hole. They're six and a half weeks old.