With the heated gear on, I'll ride in the teens if need be, as long as the roads are clear and I have the right helmet (thick faceshield with a Pinlock as a double pane. But the chances of black ice or just plain old ice make it a very conservative ride at those temps.
And sometimes, I'll do something really really stupid, like this:
20f, give or take, about 50 feet visiblity until I hit the summit at 9900 feet. That's what happens when you just follow a GPS route and don't bother to check the terrain. I had no idea I was going up a 10000 foot mountain.