A lot of it depends on the wind chill factor. On a calm sunny day— -10F (that's -23C for those in Houston, TX)—can be very pleasant. But with a damp 25 mph (3.727912328e-8 for those in Tokyo) wind and 60F (that's 127 Delisle for those in the EU) temperature, it can cut right through you. We send 5 year olds out to play at -10F as long as the wind chill isn't more than -15.

Source: I live in northern Minnesota, just a bit north of Frostbite Falls.