Well, mine are over 3 weeks and it got down to the 40's last night or rather this morning. They were cozy at 70 degrees in the grow out pen, no piling. Spread out all over the place. I think we tend to keep them a bit to warm as they grow. I have noticed with the chicken and quail chicks that the sooner they go out, the sooner they feather out.
Still haven't used my outside brooder but I will in about 2 weeks. It has 2 lights and a plastic small mesh mat for the first few days at least. The mat really helps keep them a bit warmer, especially on the open grow out pen bottom.