Most of the math and explicit rationality came later, after I learned to program, but my first step down this path was probably when I was around six. I was suspicious of the whole idea of the tooth fairy, so one night after losing a tooth I did a little experiment: I put it under my pillow without telling anyone. The next morning, I showed my parents, and they actually came clean (obviously they couldn't keep things going with santa claus or anything else like that). I think I still kept a vague sort of religion for a few years after that, though.