After a two-week extension of the Obamacare open enrollment deadline, the White House announced today that 8 million people have signed up through the health-care law's insurance marketplaces since it opened in October. That means about 900,000 people picked a health plan in the two weeks since the end of the original March 31 enrollment deadline.

When signups hit 7 million on April 1, I wrote about why that number alone didn't necessarily say whether the Affordable Care Act would succeed. The same goes for the newly announced 8 million signups.

Now that we pretty much have the final tally (a few state-run exchanges, including the District's, are still allowing enrollment through the end of the month), here's what enrollment actually looked like vs. expectations set by the Centers for Medicare and Medicaid Services in September before the insurance marketplaces opened.

You can see that signups surged in March as expected, and the two bonus weeks of enrollment in April essentially matched signups for the entire month of February. CMS never made a projection for April enrollment because it was not supposed to happen.

If we learned one thing from this enrollment period, it's this: Never underestimate the power of procrastination. California's exchange on Thursday reported that April 15, its last day of enrollment, saw 50,000 people sign up — its best day ever.

Youth Enrollment

If you tuned into President Obama's news conference Thursday afternoon, you heard him say that 35 percent of people who signed up for exchange plans were under 35. That's true, but there's a number that matters much more to the success of the new insurance markets: how many enrollees are between 18 and 34 years old. In the end, young adults accounted for 28 percent of signups.

Obamacare supporters predicted all along an enrollment bump in the crucial young adult demographic, and it sort of happened. In January and February, young adults accounted for 25 percent of total signups.

Last summer, the administration had hoped that 40 percent of exchange enrollees would be between 18 and 34, based on the Congressional Budget Office's estimates that 7 million people in all would enroll in 2014. Instead, the administration got 28 percent, which the White House and supporters very eagerly pointed out Thursday was nearly identical to youth enrollment in the first year of the Massachusetts health-care law that Obamacare was based on.

Is that rate good enough for the federal law? Probably. Previous analysis from the Kaiser Family Foundation suggests that enough young people have signed up to avoid a major spike in premiums next year. And the CBO earlier this week said it expects a small increase in premiums in 2015. Young people may benefit the most from special enrollment periods between now and the November start of open enrollment, so their numbers may still grow.

The 2014 enrollment stories aren't over, however. We're still waiting to find out at least the following: How many purchased coverage off the exchanges, how many paid their premiums, what kind of coverage they purchased, how many joined Medicaid because of the expansion and more.

So, stay tuned.