Well my family was never very religious. We went to church when me and my brother were little but stopped. No crosses, bibles, no talk of God, nada. I had to ask my mom at about age 7 what religion I was and she thought for a moment and I was baptized Catholic. Then I asked what she was and wasn't too specific, muttered something about protestant or something, I don't even know now.
Well when I was in middle school I was already putting the pieces together. I saw how religion just seemed so perfectly man-made, and I saw it was the same as the "silly" religious beliefs of days past like with the Greeks and Egyptians. I realized man had needed God to explain what they could not, and I realized science now filled that role so I found it silly to still hold on to such beliefs. God was pretty much obsolete. Even at age 10 or so I could see how ridiculous the idea of some man in the sky that was all powerful but never gave people any indicaton that he was there, other than instances in the bible, which I also had many issues with. Giants, demons, talking animals, global floods, seas parting...crazy talk. I mean, with all the different versions of Christanity out there, and them claiming very different paths to heaven, it would be impossible to know which was correct, and any god who'd hold that against someone wasn't the loving, forgiving god they were preaching about anyway.
At first I figured that, maybe there was a god but I didn't know what to do to get to heaven. So I decided, well if I'm a good person any just god would accept me. Then of course I couldn't shake how silly it all was, and the only thread that held me to any of it was the fear of hell. I knew it was highly improbable but there was that, well maybe I'm wrong, thought in the back of my head. But eventually even that wasn't enough so I was completely god and religion free by time I got to high school. Sad to think that by age 13 I was smarter at least in that sense than a lot of grown people.