I never believed in Santa; I was raised thinking Santa Claus was just a game people played at Christmas and for the longest time I thought that was what everyone thought. Same with the easter bunny/tooth fairy.
It also makes me think of the whole George Reeves/Superman thing. Did kids from the era really believe Superman was real? Do kids these days still have trouble telling fiction from reality?
I don't know if it's just an autistic thing to be able to accept things as fictional from a young age but I think it's a pretty important skill to have. I know Mr Rogers was pretty big on separating the "real" and "make-believe" sections of his show (and even sometimes broke the fourth wall on the "real" portions to help remind viewers that it was all a show).
I'm going to play armchair psychologist here and wonder if a lack of comprehending fiction stunts children's creativity because understanding that Superman is a story someone made up leads into the idea that "I can make up my own stories too".