I'm not sure I can answer exactly. I'm not sure I ever really believed. I can remember becoming a bit sceptical at around 10 or 11 - before that, I'd just been told and 'accepted' (as a young child does) the primary school teachings of a kindly god, with a big white beard sitting on a cloud in heaven.
Throughout teenage years I became very sceptical - it just seemed to me a very 'convenient' answer to all the questions we human beings couldn't answer - but that scientific discovery was making clearer to us. The more I thought about it, the less likely it seemed to me that there was one all-powerful creator - and this was the one true god whereas Thor, Zeus etc weren't (anymore).
I suppose really in late teen years/university years it was various different things - I recognise faith as a comfort to other people, but I just didn't get it. I can appreciate the wonders of nature, the universe, love, etc without needing to believe in a deity.
I remember a friend of mine once saying "is it more likely that god created man or that man created god?" and that says it all to me.
OP did you ask because you genuinely want to know? 