A closer look at the HHS report reveals the following in End Note #4 (emphasis mine):
A recent survey from the Commonwealth Fund found that 6.6 million young adults who have enrolled in their parents’ health plans since November 2010 were unlikely to have been eligible for those plans before the Affordable Care Act. This number exceeds our calculation because it includes some individuals who were already insured, often through their own private coverage. Other recent research (Sommers & Kronick, 2012 – see Footnote 5) suggests that just under 60% of the increased coverage of young adults on their parents’ plans came from individuals switching from their own private plans; multiplying the remaining 40% by Commonwealth’s estimate of 6.6 million indicates 2.6 million young adults gaining coverage, which is within range of our 3 million estimate from NHIS data.What this means is that almost four million young adults who had obtained health insurance on their own are back on their parents' insurance. At least until they are 26. Never let it be said that the Obama administration wants kids to grow up too fast.