Mine have been outside since they were 3 weeks old. They had already started to get feathers and the back porch was stinking, so out they went. They are now 5 weeks old and some nights have gotten down to the mid 30f. They are more then fine. Good Luck
If your still getting frost I would put the heat lamp out there but only turn it on when then nights are down in the 30's or your getting frost. but shut it off in day time.