Pullets not roosting?
Roosting is a very deep rooted instinct. They will eventually "get" the urge to roost. I would let them decide.
I also think roosting is a learned behavior. What I mean is that chicks that are raised with a hen will roost much sooner than those who are not. I've had hens lure chicks to the roost at about a month old.
I have 14 pullets and 11 cockerels of the same age (12 weeks) they've been in the coop for only two weeks because of weather delays here.
Coop is inside a small shed. I tried putting them on the roosts too. Right off. Built ramps and baited them with treats. They stay until the food is gone then back on the floor. I decided to let them figure it out. Two are now roosting on the actual roost bars. Three get onto the kick board, to keep litter in, across the person gate which leads to their pen and the others crowd behind the three hoping they get a turn on the board. I'm thinking eventually they will all figure out the actual roost boards.