If you would like them to begin roosting soon, you can start placing them on the roosts throughout the day when you can. They will probably immediately fly off but it introduces them to it. You could also go in during bedtime when the are sleeping and put them on the roosts and quietly sneak back out. Or you could try putting a cheap 2-3 rung step ladder, something more at their level and give them the option of different levels. They will work their way up higher as they get even older.
My chicks begin roosting in their first week, starting to sleep on their feed trays. I've moved them out to the coop this week and they are sleeping on lowlevel rungs of the roosts, but I imagine as they get older their preferred roosts will get higher.
They won't need the nest boxes until they are ready to begin laying. If I remember correctly, I added our nest boxes when our gang turned 15 weeks old. They began laying anywhere from 16 and a half weeks to 22 weeks old.
You don't want your chicks to start roosting in the nest boxes, they'll keep them a poopy mess.