We always spent Easter at my grandmother's. When I saw my Easter basket while the car was being packed (I think the black garbage bag it was in slipped or something) I confronted my mom about it, and she said there was no Easter Bunny. I asked if that meant there was no Santa Claus too, and she said yes. I cried, she cried, it was a bad scene. I was 10 and in 5th grade, and despite believing that long I got over it.
I also disagree with the fact that smart kids stop believing. Belief is what you make of it. I rationalized very early on that none of the department store and other such Santas were the same man. I figured logically he couldn't be everywhere at once, so he was sending elves to get the wishes from kids, and they dressed as Santa because some of the less smart kids didn't know the difference. I figured my parents couldn't afford my presents, which was partially due to the fact I had a very poor idea of what could be bought for a dollar, and I didn't know my parents had a little Christmas fund they put into all year. But my point is, an intelligent child who truly wants to believe will build an entire mythos about it, as I did. I simply defended my beliefs in school and I was not made fun of, though it sometimes caused me to doubt a little.
I think the major thing that made me believe that long was a family friend, who has now passed, bless him. He loved to dress as Santa for parties and church functions, but he was Greek Orthodox and they celebrated on a different day. So he had fun visiting my house a few times as Santa on Christmas Eve. And this was a man I never saw my entire life except when he was Santa. There was no way to recognize him as my dad or an uncle or a grandpa. He stopped coming for a year or two, then one year as I was really starting to doubt, he showed up by total surprise again (my parents didn't even know he was coming) and I think that got another two years of belief out of me until the aforementioned incident.
If I hadn't found out, my parents would have told me before middle school. I think at that point it's appropriate.
But just because your kid no longer believes doesn't mean you can't have fun with it. My mom sent me an email the other day saying "Santa called. He wants to know if you want anything for Christmas." And we all play along. It's something fun and special we enjoy.