So maybe 10+ weeks old. Do you have a lower roost that is easy to get up on? I usually start with a very low bar in the brooder for them to practice on and then we usually have a "ramp" leading up to the real roosting bar as this makes it easier for them to get up. My chicks have huddled together (this is how they are used to doing it) after I moved them from the brooder to the coop for quite some time. It usually takes just one to get up on the roost and the rest will follow. I have at least one or two hens that will never get on the roost. (due to pecking order) The problems that arise from not getting on the roost are mostly things like broken, soiled eggs and perhaps even egg eating at some point. It is also easier for them to have infestations such as mites. You can train them to roost by going out after dark when they are all huddled together,with a small flashlight (preferably one with a colored light) and placing them on the roost yourself. This may take a week or two but it usually works. Make sure it is really dark and they are "sleeping" and use as little fussing as possible so they won't want to just jump down right away.
CJ