Separating fact from fantasy is important even for 5 and 6 year olds. Some false beliefs are not as harmless as Santa Claus.
I guess the issue is how far fictitious beliefs go. If they are carried into adulthood, that leads to problems.
For things like Santa Claus, generally speaking, parents will have that 'talk' about Santa when the kids are at a certain age. I can't be certain one way or another if religious beliefs at a young age will automatically cloud an adults judgement. Clearly we have seen evidence of that, but I have also seen religious people who had lots of Bible study in childhood and yet they grow up with an ability to discern fact from fiction. I guess maybe it depends on how deeply one is 'indoctrinated'.
And it probably doesn't have to be just religious beliefs. A child's mind is like a sponge. Children of racist parents or parents who have strong opinions on the New World Order and evil governments (i.e. the tinfoil helmet crowd) probably may have their worldviews skewed in adulthood.