I have to agree with JDAx3 and Nautigal, my 7 y/o still believes but i think it has a lot to do with how big a deal we make of it. we go out of our way to keep the magic alive - i did it for years as a child even long after i knew and my parents knew i knew, i didn't stop writing letters to santa until long into high school! it was part of the fun of it. My boy is an extremely logical one, but he also has a wonderful imagination and he goes with it. We've also been lucky that there is a fabulous local Santa who comes to our office every year - for pics - but my son has written santa a letter every year and hand delivered it. I freaked when i found out his school would have a Santa visit too... but it was the same Santa! i really lucked out there.... at his school they also do a big deal with the mischievous leprechaun, so that helps too
There are a couple of fabulous books about Santa and the Reindeer that explain a lot about the magic, Santa's helpers, etc. Those have been a wonderful addition to our repertoire. And we have lots of stories about why different books may have different stories, pictures, etc (it's a mystery, magic often is!).
I think there is something to how much the parents want to keep it alive, frankly, that the kids sense too... I think if i'd let him, big man would have "figured it out" sooner... heck, for all i know he has deep down, but he's into it hook line and sinker, just like his dad and I... and the little one is along for the ride! (his imagination is even more active, so he's just added on to the story....)
All that said, i'm glad someone asked the question... i'd wondered the same thing myself!!