I think there is a definite point where, as parents, we transition the tradition of Santa Claus from a real guy to a spirit of the season sort of thing. Without being too judgemental, I do cringe to read about parents engaging in trickery to keep a kid believing who would otherwise be exploring the doubts of a rational mind.
My DD had it figured out at 8. DS is 2 years behind her and she was happy to play along. He figured it out at about 8. No hard feelings. They're 11 and 13 now.