At what age should a child be introduced to religion?

[arewethereyet]

Ideally, this is something that the public, secular school system should be doing, say, in a World religions class.

I took 'World Religions' at Catholic school.  
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I didn't introduce my kids to religion but I didn't shield them from it either...how could I have when it's everywhere.  I let them go to church with friends and among the three they pretty much were exposed to most flavors of Christian groups.  From hellfire and brimstone revivals to Pentecostals speaking in tongues to whatever they call services at a JW Kingdom hall.  I answered a lot of questions with "some people believe".  Yes, there was a Bible in the house and there was a children's Bible when they were young.  I had hopes that they were bright enough to come to their own conclusions if allowed to explore.  Their belief or lack of needed to be theirs not mine.