It depends on how cold it is, what sort of outdoor coop you have, and how large the chick is (age is relative, size and amount of adult feathers is more critical to cold hardiness.
I have 6 that are 9 weeks old and fully feathered that are still in the basement. They really could go out, but the extreme cold we've been having worries me. I lost a group in March of last year because I assumed they would be able to stand the cold - NOT making that mistake again. Better safe than sorry is my thought, so they will be in the basement another week. Hopefully next Saturday they can go out.
Edit to add: I've become a bit obsessed with watching the long range temp forecasts. I don't feel like I can trust them, but I want to believe . . .
Please, can spring start early this year?
I'm glad to hear this, I felt like a weenie having my chick indoors with her broody Mama still.

Mama has had it and I think tomorrow they will go out permenantly. I thought of just putting the Mama chicken out, but I was fearful she would forget her chick and that it might be bullied more. The chick was hatched Dec. 25th. I think she will be okay to tuck herself under Mama if she gets chilly. Or I mih just keep the cage handy....

@wingstone Your bantam cochin finally squatted for me today! She will probably laying fertile eggs soon since my Silkie roo "favors" her. And by favors, I mean he has tried to hop on her only.
