Via Drudge, this story from England is about a teacher who is in trouble because she told her nine-year-old student that Father Christmas doesn't exist. The teacher seemed to think they already knew that, but apparently some didn't.
Being Jewish, I'm not much up on these things, but I'd have thought that the Santa thing pretty much dissipated by age six or seven, at the latest; even if the parents tried to maintain the legend longer, I'd think that some combination of (a) the children's own intelligence; (b) children whose parents told them the truth telling their friends that Santa doesn't exist; and (c) t.v. and other media (don't they notice that Santa looks different depending on which actor plays him?) would not let it survive.
So, V.C. readers, enlighten me: when do kids stop believing in Santa?