Yesterday me and my wife went to see Bee Movie with some friends of ours and their 3 kids. Afterwards we hung out at their place for the evening (played some Wii, watched the hockey game) and their 3 year old twin girls told me how much they love superheroes.
One just randomly told me she loved Batman and the other girl looks up and says she loves Superman.
They're three years old so I know they've never seen a comic book and wouldn't know what to do with one if they did. What they do have are some The Batman dvds that one of the girls chose. They do recognize the images of the characters and that they're superheroes. They are presented these characters in multiple other media to the point where the kids know about the characters but don't really know anything about them other than the image.
Good thing I'm in their lives. I can create little nerds and geeks. Lovely.