I don't remember ever believing in Santa Claus. My family did exchange gifts with "Santa" in the "from" field, but as best I remember I always parsed this as what I might now describe as deference to a cultural norm.

Looking back on it, I think I must have mentally assigned Santa to the same class of myths that held Aesop's Fables or stories about Zeus or Coyote: entertaining stories carrying useful lessons about culture, but not to be taken literally. The children in media that believed in Santa always seemed to belong to idealized worlds, so I didn't figure I was expected to follow suit any more than I would have been expected to, say, come up with wacky schemes to get other people to do my chores.

By the time I'd figured out that it was a live cultural norm, most of my peers had grown out of it.