They look like they're about 4 weeks old. If they're using an inside brooder, I'd remove the heat for a couple of days, then move them outside. They should be fine if the night-time temperatures are above 50 degrees.
In my opinion, chicks raised in the late fall and winter miss out on some things that are essential to growing up really healthy and strong . . . lots of sunshine and lots of tender, green vegetation to feed on.