Well, the OP is a little random, but I was pretty sure pretty early on. I remember that I tried proving to my parents that Santa didn't exist, but they always told me he did and it made me a little frustrated, especially when he "left notes" saying stuff like "Your dad told Santa not to bring presents this year because you children [my brother and I] have been naughty, but once again, your mother pleaded with me to bring some. Try to behave better."
That really bummed me out and made me feel like a jerk even though I was a pretty good kid.