I am an atheist. I don't think I can be considered to have turned one at the age of 9-15 though. Hard to say though. When I entered my teenage years, I turned more into a constructivist about God. That is to say that God is a construct that should be constructed in a manner such that it will promote proper societal order.
I turned a bit more atheist at the end of high school.
Then in Freshman year of college, I converted to an extreme fundamentalist Christianity. My reasoning was that it was there, because I didn't really care, and because women on average are more religious than men. The strange thing is that while at first I was just playing the game, eventually I gave myself in to it. It didn't work out, which is strange among the lives of those who know they are to be morally perfect, but it didn't. The way I left was itself deeply psychologically scarring, and I jumped into atheism moral rejection first, and dysteleology second, which is odd because for a time I ended up basically being a satanist, and also because a lot of atheists hold to more teleology than they ever should.
And so yeah... I am an atheist, but I really turned one in college.