Our brooder is in our unheated, drafty barn, so I guess you could say our chicks are outside from day one, too. I had exactly ONE batch of chicks in my house (my first batch, of course) and I'll never do that again. Since then, I've had chicks brooded in that barn in a freak spring chill, had a broody hen hatch her first chicks March 1 in a blizzard... I've decided that baby chicks are a lot hardier than people give them credit for, and don't even take the brooder temperature any more. I just watch the chicks, and move the light as needed. In weather like we have currently, that means raising the light very high during the 80 degree days, and lowering it for the 40 degree nights. But my chicks just hatched yesterday, the little cuties.
Having chicks raised by broody hens definitely changed my mind, as I saw her take them out into the snow to forage, and all of her babies did just fine.
If they're fully feathered except for their heads, they'll be fine. Heck, my broody hen just left her chicks to their own devices the beginning of this week, and they're five weeks old. Guess she wanted to start sleeping in the hen house, and the chicks didn't follow her, so she left them and went to roost. I got to round them up and put them in the hen house for the night.
If they have all their feathers, and you've been slowly weaning them from the light, then I would expect they'll be fine as long as they have a sheltered place to coop up at night.