The coldest weather I've ridden in was -20F/-29 C. I've gone skiing at -30F/-34 C. Both are perfectly comfortable with the right clothing - in fact, I've skiied with my then 8 year old daughter at these temperatures and she had a ball.

I agree with Matchka that below 0 F/-18 C is where cold really begins. The air feels really dry when you inhale and snow and ice become much less slippery.

Paul