Chicks are typically feathered in by about 8 weeks or so. However, that doesn't mean they can sustain zero degree temps. It means they have the ability to start regulating body temperature. In order to sustain very low temps, they have to be gradually acclimated to it. This typically happens during the natural declining of temps if they are outside when it happens. During winter that is much harder to accomplish.
If you're considering putting a young chick in with adults, you may want to look around at some other recent posts where that didn't go so well. Adult are very rough on youngsters and it can get brutal very quickly.
