I was brought up in quite a religious home. As this is the status quo in my home country i.e. everyone is extremely religious, I never knew anything different. I grew up believing in God/heaven/hell, church every Sunday, prayers every night, praying whenever I needed something..etc
I think I started to have my doubts around the age of 15, and properly gave up on the idea when I was around 17. I'm 30 now.
For me it's the absolutely shocking state of the world we live in that made me completely give up on the existence of God. I felt that even if there was a 'higher power' of some sort who had engineered all of this, they were very much the opposite of benevolent(sadistic even?), and probably did not deserve my worship.
Just the constant barrage of disasters, evil, chaos, inequalities, atrocities, the absolute randomness and luck that defines our existence...I mean look around. Would anyone really want to put their name to this?
Anyway, what was your turning point? When did you stop believing in God?