To answer my own question, I personally figured out that the whole Santa story was a lie around the age of six or so, but I continued to believe in belief, that it was right or appropriate that young children be encouraged to believe in Santa Claus. I never confronted my parents about it, but we held an "I know you know I know" understanding, and I continued to prop up my younger sister's belief for years afterward. It wasn't until years later, after my sister had stopped believing, that I started to wonder why adults would want children to believe in Santa Claus, and whether their reasons for it were actually good.
I don't think it ever really encouraged me to question adults' motives so much as learning to question adults motives led me to question it. I was a bit surprised when I started to learn how traumatic the discovery can be for children, since my own realization never seemed like a big deal to me.
that I started to wonder why adults would want children to believe in Santa Claus, and whether their reasons for it were actually good.
I think that lots of people have a kind of compulsion to lie to anyone they care about who is credulous, particularly children, about things that don't matter very much. I assume it's adaptive behaviour, to try to toughen up their reasoning skills on matters that aren't so important - to teach them that they can't rely on even good people to tell them stuff that is true.
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.