Mine vary in age from 12 weeks to 7 weeks. They have been in the coop with a roost since they were day olds. They slept in a heap until the oldest was about 7 weeks old, now they all roost. The youngest have been roosting since they were about 3 weeks old, I suspect because they didn't like being left alone on the floor they followed the others sooner than they would have on their own.
I never 'placed' them on a roost, as I'm a believer that they will do what comes natural when it seems right to them.
My biggest problem has been keeping them from roosting on things I didn't want them to (this came after they started to roost, when they discovered some higher structures in the coop), like the feed bins and nesting boxes. It took me awhile to eliminate all the alternatives, but once I did, they settled on the only thing I left available, the roost they were 'supposed' to use.
I would block off the nest boxes. From what I've read (definitely not from my own experience), it's best if they think of the nest boxes as places that are good to lay eggs in, NOT as sleeping (and pooping) areas. Open up the nest boxes when they are at point of lay.
Good luck, and don't stress about the small stuff. Just enjoy.