once they have all their feathers they can be their own down blanket. They are good till under freezing. After that you want to watch they can be fine if they are used to it have a goodd coop and friends.
ditto -- i've put chicks outdoors in unheated pens as early as 4 weeks old, and they've all done fine, even in quite cold (for the bay area) temps. chickens are tough critters.