I just put 9 bantam ameraucanas and 3 cayuga ducklings, all about 3 weeks old, into my big coop today. They were just making too much of a mess in the house, they stunk so bad even though I cleaned pretty much constantly. The highs here are ranging from 63-72, and the lows from 37-50. They did very well during the day today, foraging in the straw.
My big chickens couldn't care less. My husband hung their 250 watt red lamp about 2 feet from the ground, I put their food and water right there at the edge of the light circle. When I checked on them about an hour ago they were all sleeping together under the light, I picked one up and he was very warm, lol.
They seem to be doing very well, I am pleased. So I would say as long as they have their body feathers in, you provide a heat source, and a draft-free coop or cage, chicks can go out.