When did I first doubt religion? Age ten, while attending confirmation classes to join the Presbyterian church. I had reached the " age of accountability". I asked the pastor "Where was Jesus from 12 years old? He started his preaching years later". Couldn't get a straight answer. That's when the questions started and my lifelong search for answers began, not just about Christianity but about all religions. At 70, I'm still questioning. Ancient Aliens is starting to make more sense....
I have to say 1st grade for me. Mother sent me to catholic school and the nuns were sooooo mean. i couldn't reconcile that with a loving creator. They were all so miserable as well. Made me wonder...I waffled back and forth for years, because it was what the family expected, but I finally just couldn't do it anymore.
At the age of 8 when the bishop talked to me about baptism.
As a freshman in a parochial high school, I was forced to read Bulfinch's Mythology, and I thought how absurd it was that people once believed such foolishness. When I put my own religion to the same test, it also failed. Sam Harris illustrates thus nicely when he asks Christians to think of all the reasons they don't believe in Islam. He then points out that those are the same reasons Muslims why Muslims don't believe in Christianity. Religions can't withstand objective assessment.