I live in Southeast Alabama and our temps. are in the 90's right now and in the 80's most of the night. It's 12:30 a.m. here and my computer says it's 79 degrees outside.
With our first flock of chickens, we were very careful and waited until they were almost fully feathered to let them out with no light. Now that it's so hot and we have raised a lot more chicks, we let our newest ones out in a pen (with no light) at about 5 weeks old. They seemed to be getting TOO hot in the brooder. They have been in the pen for a week and are doing very well in there so far.
It probably at least partially depends on how hot it is where you live.