Couldn't really find a link with an average income of the single individual in the USA. Here's one showing that 1.7mm individual's primary job pays minimum wage, meaning they could work more than 1 job and effectively be making more than the $15k/year that $7.25/hr gets you. It also states that 2.2mm are working below minimum wage, but the data doesn't include "overtime pay, commissions, or tips received".
http://www.bls.gov/cps/minwage2011.htm
I think ~$20k/year sounds about right for the average pay of eligible to work Americans. Could be higher though easily as well.