I find it fascinating how kids, without being told, eventually realize that Santa Clause isn't real. Is it because they never get to see him? "Mall Santas" are a common thing all over the world, so I don't think it can be that.
Different things get dispelled that changes their foundation that Santa relies on. Such as magic isn't real. Without magic, how can Santa perform his job?
As they get older, it becomes 'childish' by peers to believe in things people 2 years younger believe.
Media created for the demographic starts hinting at the possibility of parents being Santa.
They're fed enough information over the years to piece it together by themselves.