In a recent interview, Susan Jacoby described how and why people become atheists:

[U]nless you’re raised atheist, people become atheists just as I did, by thinking about the same things Augustine thought about. Certainly one of the first things I thought about as a maturing child was “Why is there polio? Why are there diseases?” If there is a good God why are there these things? The answer of the religious person is “God has a plan we don’t understand.” That wasn’t enough for me. There are people who don’t know anything about science. One of the reasons I recommend Richard Dawkins’s book, The God Delusion, is that basically he explains the relationship between science and atheism. But I don’t think people are really persuaded into atheism by books or by debates or anything like that. I think people become atheists because they think about the world around them. They start to search out books because they ask questions. In general, people don’t become atheists at a late age, in their 50s.