One time at school (when I was 8), I was with a bunch of girls around Easter time. We were planning to hide some chocolate eggs for the preps, and I kept making jokes like "oh he isn't real" -- even though at the time, I believed he was real -- then one of the girls said "shush they're not supposed to know."
After that I got ideas, since she sounded serious and I took what she meant seriously. Then I confronted my mother about the Easter bunny, and she told me it was just a lie, like the tooth fairy and Santa Claus. That's about when I found out about it, I guess.