I was always aware that Santa is more a concept than a being. The spirit of giving, and of family, with the power of Tradition at his back. Not sure if this was something I just "knew" or if it was something I was told. Did not even matter if it was "really" my parents, it always was and was not Santa as well.
Though if I did believe it on some level other than conceptual, it was shifted to the purely conceptual model (not that I would have known what half those words mean then) when I found out one of my best friends at the time was Jewish and practiced a different set of rituals than ... well, anyone else in our school did at that time, really, about 6 or 7 years old.