I will wear top of knee or an inch above with stockings, and shorter for casual denim skirts bare legged. I rarely wear dresses or skirts any more. My problem isn't my knees, but my spider and varicose veins.
I think it depends a lot on your proportions, the fitness of the legs, and how high you go and if you wear tights or stockings. I suspect different people will be more or less judgmental about it. I also suspect it depends on the motive of the wearer--if you think you are fooling people that you are young and hip--it doesn't work.
Beth at Style of a Certain Age is in her 50s and she looks great in above the knee skirts and shorts. She was featured on YLF a few days ago. I love her look from Thursday, August 13 with the leather jacket, denim skirt and knee boots.
She wears sleeveless as well, and in her blog photos. http://www.styleatacertainage.com