I never actually believed in him - my sisters and I knew that our parents bought the presents and put them under the tree, and we were just told that Santa Claus was a story that people told other kids. Given that the three of us would be awake all night on Christmas Eve (we'd pretend to go to sleep until my folks left the room, then it would be up and start feeling the wrapped presents to work out what's in there, counting down the time until we could open the presents, etc).
I remember getting in trouble at school age 6 for making a couple 8 yr olds cry when insisting that Santa Claus was just a story. I just assumed that because they were older than me, they must obviously have been told the truth by then.