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.
What do mean by trickery?
I assume things like, "If you don't behave, Santa won't bring you anything." OR, "Santa saw you do that, so now you won't get that toy you wanted." That sort of nonsense.