This is an interesting point. Personally, I pretty much always knew I was gay. I never felt the same, but at the time, I neither understood nor could comprehend the consequences of what I was feeling. It wasn't until I was about 13 that I started to come to terms with my sexuality. I believe that is a combination of genetics, and early life experiences that contribute to ones sexuality. I believe that your genetic make up can make you more susceptible to becoming gay, so events in your early life have a greater effect on you than they might on another person. A study has shown that a man is more likely to be gay, if his father was abusive or their was a lack of input from a father figure. This doesn't mean however that if these criteria are met, you are going to be gay, or vice versa.
I hope this was useful