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.
-Kim