Santa Clause does exist. He exists in the Fourth Century, in the area that is now northern Turkey, under the name of Saint Nicholas. To say that he doesn’t exist is a lie, as much of a lie as saying he has magic reindeers and a home in present day North Pole.
The obvious rebuttal is to say he existed—not exists—in the past tense. My argument is that this is not more truthful, just more ‘our-own-time-centric’. Biased, in other words, like a metaphysical prejudice. Saint Nicholas does exist in that time and place. Being biased towards our own time and place is closed-minded, and leads to illusion and even violence. A case in point: somebody once said that the world is flat, based on the fact that no one in his own time could prove that it was round. Somebody could prove it, of course, in the future. But some notable people went to prison or were tortured, or killed, because they did not cater to the time-bias (shall I say ‘timeism’?). Giordano Bruno, I hear, is being burned at the stake in 1590 for affirming the Earth's motion around the sun.
I once heard an atheist say: “The burden of proof is on the believer; I don’t have to prove God does not exist, they need to prove he does.” This is the same as saying: “Nothing exists unless we can prove it exists.” If this is true, then the universe needs to run everything by humans before doing anything. If it is true, then the world was indeed flat, and it transformed into a sphere only at the moment we could prove it a sphere. It is human-centeredness posing as reason.
Given a modern understanding of space-time, and the implied ‘time-centeredness’ we suffer from (not to mention human-centeredness), we must acknowledge a limit to human understanding. We cannot understand a theoretical being that either is an entire universe or created an entire universe (God) any more than an ant can understand algebra. The act of personifying him as an old man in a beard is dumbing him down to our level, so we can own him. Similarly, to say this theoretical being doesn’t exist is reacting against something we can’t own.
To disbelieve is as much an act of faith as is to believe. The agnostic, not the atheist, is being reasonable. Buddha said the beginning of wisdom to be able to say, “I don’t know”.
We don’t necessarily need to grow out of believing in Santa, we need to deepen our understanding of who and when (and what) he really is.