Not only am I a bit late to the party, but I don’t suppose (at least by some definitions) that I am an atheist. I believe in some conjunction of scholastic theism and absolute idealism. Can’t put my finger on the proportions, however, and I firmly disbelieve that anything like a personal deity—a guy-in-the-sky with human emotions—exists. Since that is what most people mean by “god”, I’m quite comfortable with the title atheist and self-identify as such.
Disclaimers out of the way, like many here I can’t recall a specific argument that shocked me into considering the nonexistence of god. My eldest brother was an atheist, however, and I had religious discussions with him starting when I was about ten and he eighteen. I think it was much less the arguments and more his example that drew me to disbelief; he prodded me into the right direction by bringing ideas to my attention that I would otherwise have not encountered. I don’t know when I first realized I was an atheist, but by thirteen or so I actively lobbied not to be forced into church attendance. It was a bit of a struggle but my parents were more open-minded, perhaps, than others.
(With a great deal of embarrassment I admit some quite brief, and primarily rebellious, excursions into neopaganism when I was fourteen. I got better.)
Really? Not trying to be snotty. I almost always take “atheist” to mean—and mean it myself—nothing more than that one either lacks a belief in god or affirms the nonexistence of any god. There are atheists who believe in the supernatural, for instance, and while that may be kind of weird I don’t think it’s a contradiction.
And similarly, I almost always take “agnostic” to mean that someone believes the proposition “god exists” is not amenable to proof. For many popular or facetious conceptions of “god”, cf. Russell’s teapot, this is undeniably true even if it isn’t very profound.