We just put our 14 chicks age 6-8 weeks outside with no light, and it still gets below 40 occasionally at night. The 6 week olds (Silkies) aren't quite fully-feathered, though they do have feathers anywhere exposed. (Just under their wings is still pretty bare.) If it had just been the Silkies I would have waited or put a light out with them, but I figured the other 11 chicks would keep them warm. They've all been just fine. I think it depends partly on how many there are, but I just read in another thread here that it's pretty normal for a broody to kick her babies out on their own by about 3 weeks old.