We always treated Santa Claus as a sort of game played between the adults and the children. When the children became old enough to suspect that it *was* a game, they went along with it for a while because it was fun (and also because they wanted to keep getting presents...). At a certain point they would be co-opted to play for the adult side ("Don't spoil it for your little brother."), which made them feel grown-up about it. Even the youngest has younger cousins (and possibly soon nieces or nephews), so it has been possible to bring him in on the adult side. I feel confident that my kids will keep the game going when they have children of their own.