In general, there is more income inequality among older populations than among younger populations, if only because older people have had more time to experience rising or falling fortunes.
Furthermore, more-educated groups show greater income inequality than less-educated groups. Uneducated people are more likely to be clustered in a tight range of relatively low incomes. But the educated will include a greater range of highly motivated breadwinners and relaxed bohemians, and a greater range of winning and losing investors. A result is a greater variety of incomes. Since the United States is growing older and also more educated, income inequality will naturally rise.
Thomas Lemieux, professor of economics at the University of British Columbia, estimates that these demographic effects account for about three-quarters of the observed rise in income inequality for men and 69 to 95 percent of the observed rise in income inequality for women...
Reading the whole piece, I imagine that it will spoil breakfast for a lot of New York Times readers. And columnists.
p.s. see also Tyler's post on the use of consumption data to measure inequality.