55 degrees for me. Sometimes I roll out in just shorts when it's only 50 or so in the morning.
My knees have made it to the ripe old age of 40 and endured tons of skateboarding and bikeriding with nary a problem.
I'm guessing there is some variance from person to person as to what would be the danger zone.
I can't imagine sub 45 being good for anyone on the planet.