Are we talking here about that fat man with Christmas? Never believed in him. Then again, (almost) no-one believes in him here in the Netherlands.
Now, if we're talking about Saint Nicholas, the bishop who brings around presents on 5/6 December, then it's a different story.
I don't know exactly how old I was when I didn't believe in him anymore. We still lived in the house where I was born, so I think I was around 7 or so. In the days after 5 December (it was a Sunday, IIRC) I was continuing my exploration of the house and was heading into the unknown of the attic. There I found a list with items which were surprisingly identical to the presents my sister and me got (I can clearly remember my little sister got a new bike that year). It took only a few seconds to make the connection in my head and then I stormed down to tell my parents.
They were sitting in the living room watching TV with my little sister. They immediately denied all the accusations, probably to protect my little sister (who was 5 at that time).
A few days later my mother did take me apart and told me that Saint Nicholas did not exist anymore, but that I should not tell it to others (my little sister, etc.) because they might still believe in it. Uh... OK!
And that was it. No tears, no fights. Only now I did have a secret many of my peers might not know.
I never abused my power though...
C ya!
Marty