According to the article, "the median income for all households was reduced slightly because of the aging of the population, a trend that is expected to continue. Seniors make less than those in their prime earning years. That reduced the median income of all American households about $200 in 2011."
In other words, the baby boomers are retiring. It stands to reason that the median household income would be reduced slightly on that account.
And, of course, the 1% are doing better than ever.