I got my birds only a week ago. I think 3 are 5months old and one is about 3-4months. It has been around -5ºc for the past 3 days. I heavily ventilated the coop after skimming pats article, but I later realised I overdid it when I left a bowl of water in the coop overnight and it froze. So I closed the very large opening a little and covered the inside with bubblewrap.
Im new to this, so dont know how much the cold bothers them. They seem to sleep fine, but are a little reluctant to run around in the day but it might be because everything is covered in deep snow. I let them free range in a decent size garden with grass & plants but they keep coming back near the house or hiding under the coop which is on concrete paving.
My brown hyline is the worst, only comes out for one feed but then hides for the rest of the day. Today she stayed under the coop when the others came inside when it went dark. Like I say, not sure what to make of it. None of them seem ill.