I was born into a Christian household. I was a firm believer and fundamentalist until I was 15, but I didn't really make it known that I'd become an atheist and stop going to church until I was 17. I'm now 20 and nearly six years sober. :)
It started out being a a general skepticism in The Bible, which of course is no mystery as to why I turned from it. Then I began to wonder if the believe in god(s) itself held any merit, and I just didn't see any reason to believe in it based on the lack of evidence.
From a personal standpoint, I've tried many other religions, and the only stance that felt completely true to myself was atheism. Having been through and studied all of the major religions, it's really the only position I could ever be in.