when can they go outside in winter or can they not???

I would say they could go to an outbuilding with heat.. being so young and spending their new feathering time under heat, they will not be acclimated to cold..

they will barely be acclimated to be without heat to 70 degree weather..

I am seriously doubting they would have any down at all..

I have no idea where in the country (world) you live..

 

They should be well on their way to being fully feathered by 4-5 weeks. I have broilers coming next week and I plan to put them out around 4 weeks but this is Texas. I still plan to offer some heat if they want it.

 

I had mine in a brooder for 2 weeks. They had a lot of feathers at 2 weeks already. It was cold when I moved them outside, but they did just fine w/ a couple of heatlamps.

 

I would keep them indoors in the brooder for at least 3 weeks, 4 if you can stand the smell. By 4 they should be pretty well feathered out. Get them ready to go outside by starting with the heat lamp about 16 inches from the chips, then raise it a couple of inches every few days after the first week. When you put them out, they will still need heat. I hang a heat lamp on the inside of my tractor, under the covered portion. They should be ok by 4 weeks if you have a good heat source and a way for them to stay out of the wind. I've never done any in this cold weather, but I finished my last batch at the end of Michigan November, and we got some pretty cold nights. Put them outside, keep them as warm as you can (use two lamps if you have to, but it's heat that they can't stand more than cold,) try to keep the drafts out, and be sure to keep on their water. Obviously it's going to freeze quickly, and they go through A LOT of water. My 25 birds emptied a 6 gallon waterer every day in the fall.