I have one chicken, and she is more like a pet. I had her since she was 3 days old, and she followed me everywhere I went, so she came outside with me all the time. I personally wouldn't recommend letting a chicken LIVE outside until their feathers start developing.
Chickens need to have feathers to survive outside in the cold. My mother kept our chicks in a makeshift incubator in our oven. The pilot light and an old towel kept the chicks warm. That might sound silly but chicks are covered in soft down that is soon replaced with pin-feathers. Once these little feathers start to fill out, the chickens can be exposed; The birds will now look like "teenagers". Adult chickens can survive outside in very cold climates if they can roost and keep out of drafts. I grew up on a farm and tended our chickens. We had a chicken coop that was protected with plywood on three sides. However, the fourth side was covered with chicken wire and open to the elements. Our chickens lived very comfortably as pets for many, many years. We provided them with lots of hay for insulation and roosting boxes so they did not have to lay on the cold ground. Like most birds, they feel safer up high. Coops are important because even though our chickens were "free range" during the day, we always put them in their coop at night. Foxes and other nocturnal animals will kill them.
It depends on the outside temp. And if there is a warm coop to get out of the elements. But chicks should be fully feathered before putting outside. This should occur between 6-12 weeks.
One thing you have to be careful of is that when they are smaller they are a target for birds of prey. My entire flock was slaughtered while I was at the store by a pair of red tailed hawks. They were a couple of months old.
