What’s probably happening is that the older hens are making life miserable for them on the roosts so they are looking for a safer place to sleep. Or they are transitioning to the roosts but are afraid of those older hens. They’ve got a good reason to be afraid. Those older hens can sometimes be brutal on the roosts.
Like the others said, block them off before they go to bed and open them up the next morning early when you let them out of the coop. Eventually they will mature enough to be able to make their way into the pecking order and sleep on the roosts with the adults. That’s often when they start to lay, though if roost space is tight, it can still be a problem.
When they’re young and in transition they are pretty inconsistent but most chickens like to sleep as high as they can when they roost. I wound up putting an extra roost higher than my nest boxes but lower than my main roosts and horizontally away from the main roosts to give the younger chicks a safer place to go. It doesn’t always work but it has helped the problem a lot.
If you have space, I suggest increasing roost space, either at the same elevation or a little lower and separated a bit from the main roosts. Try to give them a safer place to go. You might need to put them on this new roost after dark for a while to get them used to using it.
Good luck! You can solve this but it can be a challenge.