When I was younger, there was no low temp limit (or none I ran into), and my personal record is -8°F. These days I'm not as tough, or just value my personal comfort more, so I have limits. I won't go out into the rain if it's below 40, though I'm willing to go lower for short runs, and will race home from work if I have to. In dry or snowy weather, I try to set a limit of 25°, but will go lower depending on my mood. Below 20° takes all the joy out of riding, so that's only for short necessary runs when I don't have a better alternative.