No rotten playground bully every told me there wasn't a Santa Claus. I think I was around 10 years old when when I came to my own realization that the jolly old elf in the red suit would not be visiting this year. There was no moment of epiphany, just a gradual accumulation of incredible rationalizations to explain how it was possible for one man to deliver toys to all the boys and girls of the world in one night. Eventually they were overwhelming. One or two impossibilities like flying reindeer or navigating chimneys being explained through magic are acceptable on their own. However, when the whole Santa franchise is predicated on a sheer mass of implausible conditions, it cannot be sustained for long, even in the most willing hearts and minds.
Unbeknownst to me at the time, I was practicing Occam's Razor which states that the simplest explanation is likely the correct one. Taking all the Santa myths together, it only seemed reasonable that Christmas morning was made possible by the simplest explanation: the parents do it.
I never told my parents of my realization; why spoil their Christmas?
I don't know when my own kids figured it out. but like me, they never said anything. I'm glad they haven't because I do like sneaking things into their stockings and then denying any knowledge of it on Christmas morning.