I don't know when I became an atheist, however I don't ever remember believing in God. I always thought it was kind of "pretend". I was raised Jewish (Reform) and went to religious school through my Bar Mitzvah. My parents didn't make me continue to confirmation. While I identify as a Jew from an ethnic/cultural standpoint, I don't believe in anything supernatural.
Tim Minchin's beat poem "Storm"
sums up my feelings on the subject of spirituality.