There seems to be a bit of a cultural element in this. Runners and cyclists have significantly different cut-off points. I'd say there are some cyclists here who cover their knees at 60 F, most do by 55 F and pretty much all by 50 F. But for runners, the same things probably happen at 50, 45 and 40.

For what it's worth, if I get to select a "perfect" temperature for strenuous outdoor activities, I'd go for something between 55-60. 65 is hot.