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.
I can't really point to any event that caused me to fall out of faith, as it were...the only church I ever attended was a few weeks of Sunday School as a young boy; I rapidly grew bored with it and asked my parents if I had to attend (the answer was, "No").
Religion simply hasn't been part of my life; growing up, I was aware that it was something that was important to many people, but since no one in my immediate family ever went to church or even discussed matters theological, my only exposure to Christianity (or any other religion) was through books (I was and remain a voracious reader). As such, it strikes me as an interesting sociological phenomena, but not one that appeals to me on a personal level.