While its characteristics change over time, the concept of "coolness" remains constant: it's all about the attitude. Being hip requires personality; a bit of badassery will do the trick, and age need not stand in your way. If you're especially trendy, you'll be happening long after the moment is captured on camera, just like these 20th-century celebrities.
Just because they're in black and white doesn't make them any less cool. If anything, it's old school, which anyone will tell you is the best type of cool.
1938: Judy GarlandDoes this have the timeless cool factor?
1972: Debbie Reynolds And Carrie FisherDoes this have the timeless cool factor?
1960: Alfred HitchcockDoes this have the timeless cool factor?
1957: JFKDoes this have the timeless cool factor?