Now... 5 weeks. But they have been fine down to 0 degrees outside with heat plates at ~3 weeks, 4'x4' brooder wrapped in plastic, AND a heat lap running. That heat lamp probably keeps the brooder area at 40 or 50 degrees with a piece of plywood covering about half of the top. They've been outside since about two weeks old. Hatched Feb 20, moved outside March 8. We've had some decent cold and also had a blizzard that dropped about 8" of snow that blew all over inside there. Didn't bother the chicks at all with some plywood over the top just to make sure little or nothing blew in. Here's the weather since I moved them outside, not a perfect depiction as our lows are 3-5 degrees lower than the ones listed here because of the way our property sits. For example, the low on the 16th at our house was maybe 1 degree:
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This is the best picture I have of my setup. This brooder is inside a hardware clothed and covered run. Once the chicks are a little bigger (like for the last week now), I prop the front of this pen up on 2x8s and give them access to the ~14x14 run, but still giving them a place to be out of the wind and warm up. They've been out in the mornings in the mid-20s before the sun comes up when I'm walking down to do chores. They were 4 weeks old then. They cluster up under the heat plate at night (bigger than the one pictured now). There's 16 chicks in there. Haven't run the heat lamp for 10 days or so now.
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If you can brood them inside for two weeks it makes a huge difference in their hardiness (in my experience). When I first started with chickens 8 years ago I babied them way more than needed. They seem to harden off pretty well and are pretty smart about regulating their temperatures.
