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.