The economy contracted (technical term: recession) during the 18 months beginning in 2008 and ending in mid 2009, it supposedly has been expanding ever since (technical term: recovery). During which do you suppose American incomes declined more?
The correct, albeit counterintuitive, answer is during the recovery. Bloomberg News reports that income declined 2.6% during the 18 month contraction, on the other hand it has declined 4.8% during the three year "expansion."
We've lost a total of 7.4% of our incomes, inflation adjusted, since the recession began. That isn't an enormous amount, but it isn't what we expect of a recovery. The only group which didn't experience a drop in income are those over 65.
For further analysis, see this story in The Hill. Their view is that our declining income is a result of a labor surplus, which makes sense. Over 65s generally are not in the labor market.