I put mine in the coop in a separate, but connected brooder box at 3 weeks. At 5 weeks I let them mingle with the other chickens and they are fully integrated a few weeks after that.
If your chicks are fully feathered, they can go out in the coop.
I was wondering the same thing. My chicks are 6&7 weeks old. Dust in the house is getting ridiculous. I bring them or in the sun when is nice but some days are still 30 degrees and snow last night.