You need to make sure they are acclimated to the temps. I would say to try and keep some kind of light on them for at least 3 months, but that's just me. If they are fully feathered and used to the temps, I think about 8 weeks is okay. But you really need to make certain they are used to being in colder temps.
28 I would say when they're fully feathered. Mine went into the barn at a week old. The pullets took care of them. I gave them a heat lamp, but they never used it.
