The Long-Term Uninsured in America, 2008-2011 (Selected Intervals): Estimates for the U.S. Civilian Noninstitutionalized Population Under Age 65
http://meps.ahrq.gov/mepsweb/data_files/publications/st424/stat424.pdf
Finds that approximately 20.4 million people or 7.6% of the population under age 65 were uninsured for the four year period studied. The percentage exceeded 10% for some younger adult age groups. From the Agency for Healthcare Research and Quality