Mine , (don't re all my exact age), was at approx 12, during a car trip with our preacher. The discussion must have been something about the age of the earth, because I brought up dinosaurs. .
The preacher adamantly denied that there ever was such a thing.
That was the click moment.
Since then I've heard equally inane arguments, but once your eyes are open to their blindness, you can't go back.

I went to free school at four years old and didn't really get the god stuff there wasn't a lot of it and it was reciting the creed which I never really did learn. My parents were unbelievers along with all my wider family as far as I know. At Free school oyu can learn whatever you like so long as you're learning something and I never chose anything religious to read so I was always atheist but never had to realise it till I joined the Brownies and then got kicked out because I wouldn't go to church parade.

When I was about 10 I think. I kept asking questions, never got answers: so decided it must be total nonsense. The religion thing was just so odd to me.

When I became an atheist, I’m not sure. It’s just that I was raised non-religious but basically the kind of Christian who just by default claims that because everyone else does. Then in my 20s I became a Christian, according to the church/cult I joined. Learning the bible was exciting, because I love to learn. And when someone is always guiding you, it makes great sense. It did occur to me that if one or two biblical paragraphs needed 64-page study guides to understand them, there might be a problem. But by then the love bombing had given me so many real true friends who would never talk to me again if I left, I shoved down logic and kept with it. Until I couldn’t stand the hypocrisy. But later I ended up joining my nearby Lutheran Church and studied the bible in depth. I decided it was bullshit, but was afraid to once again lose all my friends. Until my granddaughter died. She hadn’t been baptized, and my pastor would not say she went to heaven. Did I mention she was 4 months old? And his funeral sermon basically told us all that our sin killed her. That’s when I left that church, learned a bunch of physics, and became a full fledged atheist with occasional agnostic tendencies not based on logic, but not being a Vulcan I claim the right to let emotion sway me from reason occasionally.

Now be honest? How many people read this long ass post?

I'd like to thank all the responders for their input. It's really interesting to hear your stories. I'd also like to point out that the question was quite clearly aimed at atheists, which I thought would be obvious. However, I have have some respond quite negatively toward me as if the question was aimed at them personally and I had dared to insinuate something about them.
If I saw a question like 'What does it feel like to be a woman', I wouldn't answer due to being a man. It's quite simple really.
Anyway, have a lovely evening everybody. Thanks.

I have never identified as atheist.

Probably since I was 6 or 7, and I had only started being cynical after the whole Santa thing fell apart. ( I did believe in Santa and it was fun and magical ) My parents (who were not religious and never attended church ) gave me a vague explanation of God. He made everything, he see's everything, he'll kill you and he especially frowns on playing with one's self. I immediately thought this was a crock of shit and wasn't falling for the Santa sham again. Plus this imaginary guy wasn't even cool like Santa. Santa never would've smited me for playing with myself and then watched like some perv beforehand. I told them I didn't believe in God and I don't recall any lectures, punishment or forced indoctrination. I'm grateful they handled it like they did, now my Grandparents were a whole different situation.

Why are so many posts about atheism?! This is agnostic website.

I was in a Bible-believing minister's school working on a Master of Divinity degree, with the goal of becoming a conservative Christian minister and professional Christian apologist. For years, my intellect had been screaming "Christianity is a bunck of bunk" and my emotional side desperately wanted to continue believing. I kept telling myself (and my pastors and other Christian leaders kept telling me) that if I just had enough faith, at least some of it would somehow make sense.

But then I dug deeply into Christian apologetics, and frankly, it is just one lie aftrer another. They aren't even believable lies. The vast majority of Christian apologists are simply dishonest.

I also studied the Bible with more depth than I ever had before. The deeper I got the more I couldn't avoid thinking "holy crap! this stuff is terrible, and besides, it cannot possibly be true".

So, at the end of my second year in a 3-year Master of Divinity course of study, I left seminary and Christianity, and never looked back.

I was raised Catholic. I eventually left the church when they started telling me how to vote. I tinkered with other Christian churches for a time (my wife was raised Lutheran - we tried a couple of those). I never felt any connection with it there. I investigated other religions, found none of them were for me, then made a rest stop at "spiritual but not religious" until finally coming to the conclusion that it was all BS when I actually started to put some thought into it. It was sort of a long road, but I've been happily atheist for about 20 years now.

I didn't. I'm an agnostic.

October 2014. I had been reading Waking Up by Sam Harris and I realized we didn’t need metaphysical explanations for religious/mystical experiences.

I was raised Roman Catholic. Went to 12 yrs of parochial school and 4 yrs Jesuit college. I observed the rituals, going to mass and communion almost daily. Confessing my sins (repeatiedly doing the same sins, menial sins), etc. I never felt an emotional connection. Prayers seem to be one way, internal thoughts. When I went away to grad school to the deep south (from upstate New York). I continued with church going but slowly realized that it was meaningless to me, no matter how hard I tried! My Jesuit philosophical trading (8 semesters of philosophy) developed an inquisitive mind that started to doubt all things in religion, especially the existence of a god who is intimately involved in our day to day lives. A god that creates a world where innocents suffer and die, to no fault of their own, was a not a god I wanted to “worship”. And why would a god want puny humans to worship IT. Eventually becoming comfortable with the idea that if god does exist, IT, as god is sexless, is not involved with our universe. Faith in a god is not a necessity for me. God is just a creation of our minds that demand an answer to how did this world begin. I’m comfortable with the Big Bang and science showing how random creation is.

When did you realise you were an atheist? When I woke up!

Also brought up in the UK, the change for me was a good deal more gradual. I'd never been led to believe in either the Easter Bunny or the tooth fairy, but I did believe in Santa, so discovering that I'd been hoodwinked there was quite a shock to the system. But doubts about God arose more gradually, and it's only relatively recently that I've been comfortable thinking of myself as an atheist. I consider the possibility of an Abrahamic-style god to be as close to zero as to make no difference, though I don't totally rule out some kind of creator/designer. But even that does seem a tad far-fetched to me now.