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.)