I don't remember how old I was, but I remember coming up with a great argument as to why Santa couldn't exist and then telling my parents... who pointed out the huge flaw in my argument. I remember being disappointed, but deciding that I had to go back to not believing if my reasoning turned out to be bad.

Some time later, my friend mentioned casually that Santa didn't exist. It didn't surprise me at all and I just went with it because it seemed obviously right.

I didn't know what happened, but I mentally marked it as something weird that beliefs shouldn't do, so it stands out. Looking back, it seems more like a belief in belief, coupled with the fact that I didn't trust myself to change my beliefs without understanding why I was doing it.