My knees have had a bash or two, and old age is creeping up on them (they're 86 years old now, between them, and there's a fair few thousand miles on 'em).
I can assure you that if I don't keep them warm, they hurt! That chimes with what G2EWS says.
As Baz Luhrman says, look after your knees; you'll miss them when they're gone. They may not give trouble while you're young and unblemished, but, like many skeletal things, the damage is done when you're young and you don't notice. You start noticing later in life, and then it's too late to do anything about it. Few people have such lovely knees they need to be on show all the time. If in doubt, cover them up, I say.