Corey - at about 6 weeks, they can handle 70 degrees...no lower in my opinion, altho you could get lucky. You can decrease 5 degrees per week of age to figure out where your babies are at and what they can handle. Your older ones would probably be ok, but not the little ones. Many don't truly have their full plumage until at least 8 weeks, even tho they look feathered out. I'd use caution in 40 degrees for these youngsters.
I just put 6 week old buff orps outside last week in 85 degrees and then we dropped to 40 overnight. They did not have a chance to acclimate to the change and are not fully feathered underneath, so I gave them a heat lamp for at night. During the day it reaches 70, so I unplug it until bed time again. Hope that helps.
Jody