My mom told me when I was 7, because I told her the kids at school were making fun of me for believing in Santa. I was so sure I was right, and when my mom told me the kids were right, I was shocked and crushed.
When I have kids, I'm going to let them believe for as long as possible. I still refuse to fully let go of my belief in Santa, lol! I mean, he IS real; he's just St. Nicholas, and he lives in Heaven--not the North Pole. But he's gotta still love Christmas and probably works Christmas miracles here and there!