By 8 weeks they are feathered out and ready to go BUT as they get bigger, the get smellier and make bigger waste so if it is warm enough they could go out sooner. My first batch went out at 6 weeks but it was summer and HOT! my second batch went out right at 8 weeks but they were restless in the brooder and needed more space, my third batch went out at 10 weeks because it was so cold outside. I think it is important to keep them out of drafts - this last batch are cochin bantams and seemed smaller but they were very happy to get out to the big bird coop. - They went up the ladder the first day, The second day they needed a lift but they are getting it figured out and my 2 little roosters are crowing.
I should state that they have those little downy feathers and when it is cold those feathers plump up under the bigger feathers like a down jacket creating lots of air pockets which are warmed by body heat - so long as they are out of the wind and wet they are quite warm and snuggly. They can sit on their feet and hide their faces under the wings. So once they are all feathered out - they are equipped for warmth and weather.
It really depends where you are. I live in a very "temperate" climate where it is rainy and cool all year round, so at about 2 months I usually introduce them to the concept of a coop, but not until 3 or 4 months do I let them venture on their own, as chances are there's already other adult chickens around, and they could get hurt or too scared. It isn't exactly the weather that worries me, because by 1 month of age their brooder is expanded to a point that they have a "natural" temperature on one side and the heat lamp to warm them on the other. By that age, they spend most of their time in the natural temp side. (average temp out here is 45 degrees, anything below is winter cold and anything above is summer heat. I'm talking only 5-10 degree difference.)
i am getting 60 meaties next week, and i'll keep them in the basement with a red heat light for about 2 weeks, then take them out to the coop where I'll block off an area with hay bales and put the light on them there with straw for bedding . If it is colder - I'll use 2 lights. I did this last year, and it worked well and they didn't get cold or sick from it.
It definitely depends on your set up and where you live.
Mine go out around three weeks old (even though it's still pretty cold around here), but they have their own small coop off the main one (we call it the maternity wing!). It has electricity for a heat lamp, and has its own separate small, covered run so the grown girls can check them out, but can't get to them.
If we didn't have this set up, we'd probably be putting them out a lot later.
I took mine out of the brooder at 4 weeks. Well- they sort of demanded it, cause I didn't have a top on it and they kept flying out! But they are in the chicken coop and away from the bigger birds. I had a heat lamp on them up until a couple of days ago but it go up into the 60's and I turned it off. They seem to be doing just fine. If it gets real cold again at night I may turn it back on.