From The Atlantic's Jan/Feb 2009 issue (p. 77):
[T]he median household income in the United States is $48,000... [but] the median-household-income statistic is too blunt an instrument, because it contains households headed by 20-year-olds (i.e., students), as well as 90-year-olds (i.e., retirees). If you earn $48,000 at 20, you're doing fine and don't need government help; at 90, you're on a fixed income and have different needs (and more options) from government than someone younger. According to an analysis by The Third Way, the median household income for people ages 25 to 60, the prime working years, is about $68,000; if they're married, it's about $78,000. If both spouses earn income at some point during the year, the number rises to $85,000.
Makes you reconsider the definition of middle-class, doesn't it?