I believed for a while- my parents would even ring sleigh bells to get us to go to bed on Christmas Eve (Santa knows if you are sleeping). "He's coming! Hurry up!!"
But then, my dad also thought it was hilarious to tell us that he had just heard on the news that Santa fell off somebody's roof and broke his leg, so there'd be no Christmas this year
I don't know when I stopped believing, exactly...it just sort of faded out. Maybe because I always had a little seed of doubt.