If they are fully feathered I'd guess five weeks or more. Most are fully feathered around 4 weeks but some are slower.
I've had 5-1/2 week old chicks go through nights with no supplemental heat with the lows in the mid 20's F. Where they were had good ventilation up high and good breeze protection down low where they were. Mine were also acclimated. My brooder was in the coop. One end was kept warm but on cold mornings the far end may have ice in it. So cold was not a shock. Once they get a little age on them they play in that cold end and go back to the warm side to warm up.
If yours are feathered out I don't see them having any problems in those temperatures. But I would want decent ventilation and a way for them to get out of a breeze if the wind were blowing.
It's good to see an expert's suggestions. Those checks are good to make no matter what you use for a heat source. Heat lamps, heat plates, heating pads, emitters, whatever you use can all start fires, especially of the wiring is faulty. I use a heat lamp in my brooder in the coop. I adjust the wattage based on time of year and what temperatures they are facing. Too much heat is as dangerous as too little, if not more so. When I install my heat lamp I toss that clamp that comes with it, I consider them unreliable. I hang my lamp with wire. I do not use string that can burn or plastic that can melt, but real wire.