I moved mine out at 4 weeks because there are 10 of them and the risks of being cramped inside seemed to outweigh the risks of going out. What I did was surround the perimeter of their outdoor pen with straw bales, put a bit of plywood over one of the corners as a hiding nook, and hung a 250 watt bulb from the ceiling of the pen. It was in the low 40's and unexpectedly windy when they first went out, but with the straw bales and the hot light, they were just fine. I put the light on a chain so I could lower it at night and raise it during the day.
I'm glad I moved them out, not only because they were getting cramped inside, but they are also feathering out at somewhat different rates and the large space outside allows those with more feathers to move further from the heat, while those who are still a bit fuzzy can stay right under it if they want.