There are large groups of people who live below the poverty level, mostly living in rural areas. These bring the national average way down. In Appalachia people make as low as $8K a year. How do they live off of so little? A little farming, a little hunting, a little fishing, some trading etc.
You're better off finding out the average wage of a city. Like New York, the average wage is $70K, Houston TX is $60K and so on.