I put mine out at 6 weeks this last time, simply because the weather was going wonky and I had the extra room (and I only had two of them to go out). I let them run around the yard that day, then put theminside the coop in the afternoon - so they could get used to it before the big girls came in.
They're fitting in well - they don't hang with the big girls, but the big girls run all over the yard - the two little ones stick close to the coop with BC, my 3 month old chickie who doesn't hang with the big girls either - I'm glad BC has someone to hang with, herself, now, she looked so lonely wandering by herslef (her choice).
I have hatched out chicks under broodies, and seen them out running arond in cold weather with mama - I think the babies are hardier than we want to think they are. Although, if raised inside, they do need to acclimate a bit before being plunged into really cold temps.
If there is more than one, however, and your coop is draft free and dry - they'll huddle up and be fine.
BC was pretty alone there, for awhile, and I worried about her. She was brooded here until she was 5 weeks old, then went home to her "Mama" then came back here a month ago or so, because her "Mama" got out of chickens, and asked if I wanted the spoiled one back. Heck yeah, I did
The poor girl was just confused about who her flock was, and intimidated by my girls who she'd never met before.
meri