I was just over 5 when I stopped believing, and that was an extremely traumatic experience, I tell you! Kids at school had been saying "it's the parents, there's no Santa", and I kept pestering my parents with questions to know the truth .... and they told me, on Xmas day ! I was devastated, and scared, as the world suddenly appeared to me as it has turned out to be indeed: a cruel place.
I was so traumatised by this, that I did not want my kids to believe in Santa; however, THEY kept telling me I was wrong, that Santa existed... they believed TV and other kids and the whole society instead of believing me.
My son is now 13, and he stopped believing around 6 I think, but he was fine; just said "well, so long as I still get the pressies"!
My daughter is almost 7 and still believes; she is a staunch believer too! She asked me "the question" and I did say oh-so-kindly that yes, Santa is imaginary, but she can't be ready yet: she didn't take my answer seriously!
She is so happy to believe, maybe after all it is for the better. She has just started hearing about Jesus Christ (again, from other people, as we are not particularly religious at home), so HE might take Santa's place in her heart, who knows?