At 11 years old.
I was never really into it, but at that point I clearly realized that there is no God or anything of the sort. And that, even if there is such an entity, there still is no need for organized religion per se. As if an all-powerful, all-knowing creator of the universe would give a **** what anyone does.
never questioned that decision since, it's just that growing older I found more and more reasons to not be religious.