Young adults, long the group most likely to be uninsured, are gaining health coverage faster than expected since the 2010 health law began allowing parents to cover them as dependents on family policies. Three new surveys, including two released on Wednesday, show that adults under 26 made significant and unique gains in insurance coverage in 2010 and the first half of 2011. One of them, by the Centers for Disease Control and Prevention, estimates that in the first quarter of 2011 there were 900,000 fewer uninsured adults in the 19-to-25 age bracket than in 2010.