Yes, that’s right: The overall age mix for the Affordable Care Act is virtually the same as the age mix was in Massachusetts. More important, it vindicates the predictions of experts like Gruber who said, all along, that young people would be among the last to sign up. “To get to 28 percent overall, there had to be a lot of young people among the late enrollees,” says Larry Levitt, senior vice president at the Kaiser Family Foundation. “That also bodes well for who is likely to sign up next year.”

Age is not the same as health, of course, and the state-by-state breakdown matters a lot more than the national one. And even the enrollment numbers don’t tell us as much as commonly assumed. People can sign up for insurance only to discover it’s less comprehensive than they need—or that it doesn’t provide access to the doctors and hospital they like. Truth is, we’ll need a lot more information and a lot more time before we can determine whether, as hoped, the health care law is improving economic security, increasing access to medical care, and making the health care system more efficient.

Still, the recent news about Obamacare has been far more encouraging than seemed even remotely possible a few months ago. Several independent organizations conducting survey research—Gallup, the Rand Corporation, and the Urban Institute—have found that the number of people without health insurance is dropping. It may not drop as low in 2014 as the CBO had predicted. (My guess: It won't.) But it will still amount to millions of more people with health insurance. "This means the critics' daily predictions of ruin were wrong, as usual," says Len Nichols, the health economist at George Mason University. "People signed up, both those who need care now and those who know they might need care tomorrow. There's still much to do to make the whole system sustainable for all of us, but these numbers are definitely a major step in the right direction."

The outlook on premiums is also uncertain. Underlying medical costs appear to be rising. A spate of reports has suggested that insurers were preparing massive hikes, beyond the usual year-to-year increases. But some industry officials and consultants have indicated that the increases are more likely to be modest—and that the markets, as a whole, appear stable. There’s bound to be variation, from market to market and insurer to insurer—with some good news and, yes, some bad. But the enrollment news is an encouraging sign, particularly since the law also has some built-in shock absorbers to help insurers cope with a year or two of skewed enrollment. “Once again, we’ll have to see where the rates come out, which we really won’t know comprehensively until the fall,” Gruber told me on Thursday. “And once again we have to remember, they’ll go up. They were going up by ten-plus-percent a year before the ACA. But to match Massachusetts on the percent of young people bodes very well. It’s an outstanding outcome.”