I brood outdoors from day 1. As long as they've got their heat source they're fine.
They abandon the heat on their own between 3.5 and 5 weeks depending on breed and weather.
I keep going out on 45F mornings and finding my Easter-hatched babies all clustered around the wire wall looking out at the green world.
I haven't seen them under the brooder plate for more than a few minutes during the daytime.
At this age they are fully-feathered and giving them any heat at all is actually counterproductive because it's preventing them from acclimating appropriately.
They really should have been completely off heat at 4-6 weeks and will be just fine outside without heat. :)
At this age, hot...