I voted 31-40F. My record is -2C (28F) when I had a warm garage to safely warm up the bike. I rode only in the city and kept the speeds below 60kmh (37mph). I live in Estonia and I'm pretty used to cold weather in general and I didn't have any heated gear. I used dualsport helmet without visor and some special goggles for riding.
Usually I start riding when it's at least 3 or 4 degrees Celsius.