Assuming you ever believed in god or gods at all, that is.
Poll coming in a moment. It is aimed at persons who are now or at one time in the past considered themselves an atheist or agnostic, so I am deliberately leaving out options for always & forever theists of whatever stripe. Space did not allow the poll question to be as long as I’d wished; please consider “stopped believing” to include “realized that I did not believe” and other such variations.
I remember exactly where and when and how old I was.
I was eight, walking home from the 3rd grade. I walked home alone, and so I suppose I just let my mind wander.
Somehow I got to thinking about god, I guess. I don’t remember what led up to it, but I do remember suddenly having an “Ohhhhhhh” moment and I consciously thought to myself, “God doesn’t exist.”
Things have been much better since 
Dunno if I ever really did, but I was somewhere around 13-14 when I first said I was an atheist.
All I really remember was that I was in high school.
I remember sitting in church when I was very young - maybe 6 or 7 - and realizing that I didn’t believe a word of it and couldn’t understand why the grown ups did.
My parents are atheists and we never talked about religion or god when I was growing up. It never occurred to me that I should believe in god. I was probably nine or so when I realized that some people are actually religious and go to church, and that it wasn’t something that only happened in books, like snow and calling people by honorifics.
When I was in college, I actually made a conscious effort to believe in god, but it never took.
Pretty much my experience.
Eight or nine. My parents never made any kind of effort to teach religion, but my grandparents did take me to church on Sundays. Which I loved because we got to eat lunch at the local airport afterward. Despite never praying, talking about religion, or going to church herself (weddings & baptisms aside) Mom was upset when she found out I didn’t believe and decided “Well this family is going to start going to Church”. Dad flat out refused to get up early, and it took Mom 3 Sundays to come to the conclusion that “this is a pain in the ass, believe or don’t believe whatever the hell you want”.
I don’t think I ever really believed. My mother always told me God was real, and I didn’t want to think she was wrong, but I can remember being very young and thinking things like “why does Mom want me to say prayers before I go to sleep? It’s not like there’s really someone up in the sky who is going to hear it.”
An aside: My maternal grandfather died a couple of years before I was born, and my grandmother remarried. A couple of years after my step-grandfather died - so I would have been seven or eight - I asked my mom how the whole “heaven being perfect” thing would work after my grandmother died… how is she supposed to be with two different men up there? It was an honest question, but my mother got really angry with me. She gave me a typical Christian non-answer along the lines of “it’ll just work, ok?” That was my first experience with religious evasiveness. 
I don’t remember ever believing but i have a clear memory of the first day of junior school (=2nd grade in US, i guess). I formed my hands to pray the way we were taught the year before and Graham Burton told me I was praying wrong. Big kids were supposed to intertwine their fingers when they pray. ( i think the “duh!” was implied )
“well that’s silly” i thought.
And that’s been my attitude ever since. It’s all rather silly.
I went to sunday school so my parents could watch football and it didn’t make sense to me then. I knew I didn’t believe when I went to awana and a counselor took me off in a corner to “pray” so I could be saved and all I could think of when I was reciting the words was how ridiculous I felt and did it matter that I didnt really have my eyes closed all the way.
I went through a religious phase when I was in middle school. I believe I began to doubt around sophomore year. By the time I graduated high school I was definitely agnostic/atheist(I floated between the two depending on the day.) I am firmly an atheist now, of course.
I never really thought about it as a kid, and by the time I thought about it, God seemed a pretty silly concept.
When I was just learning how to read (3? 4?), I tried to read anything with words on it. When I read my father’s dog tags I discovered a strange word stamped there, “Atheist.” When I asked what that meant, it clicked in my head that that made total sense and all of the attempts my mother had been (and would continue) making to raise my sister and I as Catholics was wasted.
Later on in my early teens I went to a Christian based sleep-away camp for 3 weeks and came back a bit brainwashed, and it took about six seconds of proselytizing away from the confines of the camp to snap me back to reality. Went back to that camp again for the next couple of summers, but I went in much warier of their powers of persuasion and with tough questions for the counselors.
Like Santa Claus, the Easter Bunny and the Tooth Fairy, I figured out it was the same old lying ass bullshit and was done with it by 2nd or 3rd grade. Growing up in the Bible Belt reinforced it, big time. As a 9 yo, I was flabbergasted that these adult people whom I was supposed to respect were either that stupid to fall for it, or were lying about falling for it.
ETA: that’s how I felt as a kid. Now I’m all grown up, I’ve learned to respect (some) people of faith, the walk-walkers, as it were.
I never gave any thought to it, my parents (athiests) gave me a children’s bible to read when I was 6 or 7, and asked me what I thought of it. My analysis was recalled to me many years later by my mother. I simply stated, “Well, I liked some of the fighting, but it’s a bit silly, isn’t it?”
I can see much greater logic in an invisible pink unicorn or the flying spaghetti monster tbh.
I first heard of God when I was maybe three or four - I was going to a Christian preschool. I remember coming home one day and asking my mother “Who’s God?”. She answered “God is a person some people believe in…”. So yeah, never any belief here.
Somehow I spent the next five or so years with the idea that God looked just like Julius Caesar, from the Asterix and Obelix cartoons - but made out of clouds. I have no idea where that came from.
The exact ages are rather fuzzy, and I was never much of a church-goer anyway, but I lost my belief in my mid 20s. I became a Theist in my mid to late 30s.
Shed Catholicism in favor of agnosticism when I was 17. Realized that my agnosticism was really just prettied-up atheism when I was 32. Answered the poll with the latter.
Mid-thirties. I was even a (liberal)Christian when I joined up here. It was my quest to learn all I could about my religion that made me lose my faith in my god. After dabbling in a few other religions I realized they were all just as unbelievable.