This morning Nemesis linked to a really interesting article. I read the whole thing and thought it was well put together. Also, I've heard other theories like this, but in this article the author is sussinct and clear about his point.
Kay Hymowitz: The child-man
In case you are too busy to click over there (believe me, I understand. I hardly ever click over to read people's links), the gist is that single white males these days are allowed to dwell in an immature, adolescent limbo between being a teenager and becoming an adult.