What? H-how could you say something like that? Oh, I get it. This is a joke ... a big, fat ... hairy joke! Right? I mean ... Santa exists! I mean, he's at the mall every year! What better proof is that?!
Seriously though, I figured it out myself when I was 10. I had my doubts before that, but what really did it was when I found the letter I wrote to Santa crumpled up under my parent's bed. They later confirmed that I was right. 😀