When Do Kids Stop Believing?

I always wondered about this:

Most children start questioning the existence of Santa Claus when they are between 6 and 8. By the time they are 9 or 10, most have ceased believing entirely, said Paul Harris, a developmental psychologist and a professor at Harvard University.

Link.

My 12-year-old doesn't believe. He said he figured it out when he was nine. He thinks his sister (11) still believes (or doesn't want to admit she has figured it out) and says all the others of believing age (9, 7, 4) still think the big guy comes around on the 25th.