Probably around 9 as well, I was one of the last kids in my peer group to really hold on to the belief and I think a part of me knew it wasn't real, but I was a very sensitive child so I clung to it pretty fiercely and got into screaming matches with anyone at school who tried to tell me otherwise.
That year I had trouble falling asleep and was still awake when my parents started moving all the "from Santa" gifts out of their bedroom, which was right next to my room, and down into the livingroom to put under the tree. That sealed it for me and I remember being very upset and crying myself to sleep that night.
Good god, that sounds depressing. D:
Oh my god, I want to hug your little babby self. That gave me feels.
I really can't remember when I stopped believing, probably under 10. I do however remember pretending to believe to get my little brother excited for Christmas, he was only 4 at the time.