10ºF is the limit that my son's school has in place, which seems reasonable to me. I think when I was growing up the limit was 0.

Seriously, I am starting to think people south of the Mason-Dixon are kinda wussy. I have friends in TX and Alabama and they whine when it gets below 50 that it is too cold to go outside (most runners will agree that 50 is ideal for running). Then all Summer long they whine that it's too hot. These are people who use treadmills more than 6 months of the year and think I'm nuts when I go running in below-freezing or even sub-zero temps with ample snow on the ground. I think they are nuts for being willing to run or bike indoors when the roads are dry and temps are not dangerous in either extreme.