I figured it out when I was 6. One day I started to think about Santa flying around and it hit me. Hey, wait a second. That can't possibly happen. My mom was crushed that I figured it out. But I believed in the tooth fairy for a couple more years after that.
Anyways, I was proud of myself for figuring out about Santa.
My wife and I never let our kids believe that Santa was real, but they still got a gift from "Santa". One of them chose to believe in Santa anyway and reasoned with us, when we would open presents on Christmas eve or boxing day, that Santa gave us his presents. This is the same kid who was opposite of me, and he never ever believed in the Tooth Fairy. When he lost his first tooth he said "Can you just give me the money now? The Tooth Fairy is phony."