My chickens are around 12-14 weeks of age and they aren't going in their coop when it gets dark. They are usually huddled in a corner of their run. I'm thinking about putting a light in their coop but I am not sure if it will work or not. Thanks!
Sometimes a night light helps. I just round mine up for a few weeks. Young birds often wait too long in the evening before going in than they are afraid because the coop is too dark.
I'm no expert here, but I did keep them in the coop for a week or two before allowing them to come out. It was still very cold and snowy here, so this may have helped. Good luck!