I don't remember a clear time when I stopped believing in Santa Claus, but I do remember some of the hints along the way. I especially remember how my parents would ask me what Santa Claus was bringing for Christmas and giving them coy answers, to see if Santa could know what I wanted even if I didn't tell him.
It didn't bother me whether or not Santa Claus was real, and I played along when my sister asked my parents. I knew who the real agent behind Santa Claus, was, though, and in third grade made sure to carefully explain to my parents why Santa should bring me chickens for Christmas. I couldn't maintain plausible deniability any longer, though, when my father took me out to clear away ground for construction of a coop on Christmas Eve.
I've long entertained a dubious regard for the practice of lying to children about the existence of Santa Claus. Parents might claim that it serves to make children's lives more magical and exciting, but as a general rule, children are adequately equipped to create fantasies of their own without their parents' intervention. The two reasons I suspect rest at the bottom line are adherence to tradition, and finding it cute to see one's children believing ridiculous things.
Personally, I considered this to be a rather indecent way to treat one's own children, and have sometimes wondered whether a large proportion of conspiracy theorists owe their origins to the realization that practically all the adults in the country really are conspiring to deceive children for no tangible benefit. However, since I began frequenting this site, I've been exposed to the alternate viewpoint that this realization may be good for developing rationalists, because it provides children with the experience of discovering that they hold beliefs which are wrong and absurd, and that they must reject them.
So, how did the Santa deception affect you personally? How do you think your life might have been different without it? If your parents didn't do it to you, what are your impressions on the experience of not being lied to when most other children are?
Also, I promise to upvote anyone who links to an easy to register for community of conspiracy theorists where they would not be averse to being asked the same question.