I put my girls out at 6 weeks, with no roost in the coop knowing full well that they wouldn't need it right away.
I did put a big 4x4 post on the floor of the coop for them in case they wanted to get up out of the bedding, but for the most part, they preferred to pile up on each other in the corner under the hanging waterer I have in there with them. The following week I went out into the woods behind my house and found a suitable "log" and trimmed it up to put up in the coop for a roost. When I put that up, I also put up a bunch of branches in the corner of their run at different levels. The highest branch was about the same height from the ground as the roosting log was from the floor of the coop. At first, none of them could actually make it up to the highest branch in a single bound, but they had all kinds of fun during the day hopping up there from the lower branches... until finally they figured out that those two flappy things on either side of their bodies were actually helpful in getting up there in a single bound. But still, for about 2 weeks they would just huddle in the corner of the coop at night just like they were doing in the brooder box. It was like they didn't even see the roosting log in the coop.
When they were 8 weeks old, I had just refilled the waterer in the coop, and my leghorn came into the coop to investigate... Suddenly... her head cocked to the side and she took on a general, "whoa... there's a log up there" expression... She finally noticed the roosting log in her coop and by golly, she was going to get up there to investigate that thing. She had all that practice out in the run, on those branches, and hopped right up there, no sweat. Well, let me tell you... when she did that, one of my Wyandottes and one of my Easter Eggers were in there... and it blew their little minds. Once that leghorn got up there, it wasn't long before the others wanted up there and sure enough... they all made a point to get up there on that roost... but they just were not interested in sleeping on it. They would always get back down and huddle in the corner for the night.
They did this for a whole week... until one night... one of them, jockeying for a better position I am sure, stood up in the middle of that pile and tipped that hanging waterer above them just enough to splash everyone. What a commotion! That night, rather than sleep in the wet spot, they all hopped up on that roost... and they've been there every night ever since. Interestingly, when they started roosting up there, they also started getting the hang of going in the coop for the night on their own. I have one of those automatic doors, and it literally wasn't until last week that they stopped lolly-gagging around and missing their chance to go in before that door closed and locked 'em out.
In another 6-8 weeks they'll start laying eggs for me. *sniff* They grow up so fast...