The Upshot has a detailed, chart-filled summary of the shrinking middle class, categorized by age, education, race, and family status.
The middle class, if defined as households making between $35,000 and $100,000 a year, shrank in the final decades of the 20th century. For a welcome reason, though: More Americans moved up into what might be considered the upper middle class or the affluent. Since 2000, the middle class has been shrinking for a decidedly more alarming reason: Incomes have fallen.
The key is that post-2000 mark. Middle income consistently down, but lower income up and upper income down.