I don’t know that I was ever turned “off” from Christianity...I never had a negative experience with it, but was never particularly religious. I was an agnostic by age 18, and an atheist by 30 (and remain so).
Perhaps if I’d been brought up in a household with stronger religious traditions, things would have turned out differently.
In my case it was certain life events followed by the realization that neither of my parents believed(despite their insistence to the contrary). Both occurred before I was 13.