I think I am going to put mine out in the tractor at 3 1/2 weeks - 4 weeks old. I have pretty much taken all the heat off of them at 2 weeks. They are in my garage and it is plenty warm in there. If I notice them huddling, I turn on the light for them.
Again though, I am babying mine. I am feeding them whatever they want whenever they want, and after dealing with them and pullets of the same age, I would rather keep the meaties and dispatch the pullets. The cornish are such big babies. They greet us every time we come out in the garage. The pullets act like maniacs and will not even come close to us.