When I was younger and lived in a snowy area I would ride until they started salting the roads. I lived in an apartment and could not wash off salt well and my bike was my only transportation so I wouldn't risk it

Now I'm more like rdtompki...I might ride in the 40's but only if I'm pretty sure its going to warm up during the ride. In my defense (other than I've moved to a warmer climate and therefore am less hardy) I do only ride in shorts