According to this analysis of data in California, Illinois and South Carolina, youth aging out of foster care are under-employed, earn significantly less than their low-income youth counterparts, and have mean earnings below the poverty level.
http://aspe.hhs.gov/hsp/fostercare-agingout02/index.htm
Connect for Kids' foster care topic page has more information on youth who have hit foster care age limits, and what's being done nationally and locally to help them on the road to independence. (See the blue sidebar for sub-topic sections.)
http://www.connectforkids.org/resources3139/resources_subject.htm?doc_id=82343