I was a very analytical kid. My mom tried to get me to buy the whole Santa thing, but we did not have a fireplace (and what about other people who didn't have fireplaces?), and the whole explanation went downhill from there.
When I was in kindergarden, Santa came to our school in a helicopter, but I still wasn't buying it. Where was his sleigh? Why was there a different Santa at every mall? I played along for my younger sisters and brother, but I never bought it myself. I could never find the presents though. My parents were too far ahead of me.
One year, I sought to prove the lack of existence of Santa. I was about eight. I hanged out under the Christmas tree. I fell asleep and woke up surrounded by toys, but I still did not buy it. Like I said, my parents were pretty slick in that department.
Christmas was always a great and blessed event for me (helping others and spending time with family), but I just never believed in Santa.
If my kids wanted to believe in Santa, I would tell them the whole story when they were ready, but I wouldn't try to force the myth of Santa onto them.