I don't exactly know how it came about - but my 10-year-old believes in Santa but he knows the gentleman in the red coat is a legend/fairytale.
About age 7 we started choosing an "angel" to shop for and really getting involved in homeless missions,etc., during the rest of the year. As my son began to see how we helped out others "secretly", I guess he figured out the "truth". All with a little help from the more cynical of his school chums.
However, we still get him the letter from Santa complete with NORTH POLE postmark and this year he got a phone call from Santa! The Caller ID said, "SANTA'S CALLING" and after he spoke to Santa for a sec, Santa played a recorded message from Mrs. Santa - Twas the Night Before Christmas. Though he knows we do things - he loves the mystery, suspense and surprise. And, like us, I think he really enjoys the fantasy!!
Most of all though - this year - he got to see the children's faces at the Ronald McDonald House when he helped distribute their presents under the tree = THAT was enough to convince him that this Santa "stuff" is pretty darn important!