I don't remember a time when I wasn't in some sense interested in rationality in some sense... but I can remember one time being at a bookstore and seeing Bertrand Russell's "Why I Am Not a Christian" (this being back when I was one) and thinking "Maybe I should read that and see what the other side says." I came home with it and my mom saw it and asked why I would want to read that when it might make me doubt. I clearly remember thinking about it and responding with something along the lines of "If you don't know both sides, how could you possibly know which one is right? Wouldn't you rather be right than keep the same wrong beliefs?" I don't know that it was a turning point for me, but it was the first time I had really had that thought out loud (and it was probably the start of my deconversion and subsequent start down the road of rationality).