The uninsured rate for American adults has fallen to its lowest level since the beginning of 2008, according to a Gallup survey released Monday. The 13.4 percent who lack health insurance is down from a peak of 18 percent in late 2013. Uninsurance rates among blacks, Hispanics, and the poor experienced the steepest decreases. (The latter two of those groups still had uninsured rates higher than the national average, with a third of Hispanics lacking insurance.) The uninsured rate for ages 18 to 25 fell 4.5 percent, suggesting young adults have been enrolling in plans through the Affordable Care Act just as the White House hoped.