Well, what are your boundaries on "consider"? I remember entertaining the possibility of God's existence at a pretty early age -- somewhere between two and four, but infantile amnesia's eaten the specifics -- but always as a hypothetical, a playing-pretend game. It took me quite a while to realize that my peers weren't consciously participating in a pleasant fantasy; as late as age eight or so I remember making decisions that could only have been predicated on the opposite assumption.
For context, I was raised in a pretty obviously Christian cultural milieu -- picture books of Bible stories, a sense that it was normal to go to church on Sundays even if you and your parents didn't -- but most of my very early authority figures didn't make a conspicuous show of belief. "Secular" might be the word, but only implicitly so.
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).