As far as I remember I think I just realised it was stupid one day. I don't even remember believing in him, the earliest Santa related memory I have is talking with my friends when I was about 10 or 11 about how silly it was, though the friend who was a year younger was convinced she had seen him deliver her presents the year before.
Actually, I remember my grandparents telling me that he knew they were my grandparents so that's why he left some of my gifts at their house. I think I was already suspicious at that point though, and that can't have been after I was 9.
