Standard wisdom that I've always heard is that you should cover your knees at anything below 60 degrees. I'm not sure where that originated, but you see it in a lot of articles. The reasoning behind it is that all the ligaments and tendons in your knee are so close to the surface that they are easily impacted by the cold and become more susceptible to injury.

The bottom line for me is that I would prefer to be warm than cold, so I tend to overdress when the temps are "between" cold and hot.