When Is Belief in Santa Claus Rational?

We can start from the assertion that anyone older than 18 cannot rationally believe in the existence of Santa. Is it ever rational?

For a five year old, you have a number of ways of acquiring this as a reasonable belief. First, there is a seemingly legitimate argument from authority. Your parents, who have been right about many things, who have more experience about the world, and who get along in the world sufficiently well tell you that Santa Claus is real. There is direct empirical evidence -- you see someone who look s like him in various places and finding folks dressed in red fur with a white beard is unusual enough to rule out chance. One could follow the hypothetico-deductive method which many argue is the foundational for legitimate scientific reasoning -- start with a hypothesis, deduce a prediction, test it, if the test is possible claim inductive support for the hypothesis and test again. Make a list of presents you would like, mail it to Santa at the North Pole, wake up in the morning and see if any of those presents are there. Successful runs of the experiment each Christmas seems to provide evidence.

So, if we argue that it is rational to believe at five, but not at eighteen, what piece of evidence or independent inferences in the years between turns the belief from rational to irrational?