In a remarkable rebound from the botched rollout of Obamacare, 8 million people have signed up for private health insurance via the exchanges created by the Affordable Care Act since October, President Barack Obama announced during a press briefing at the White House Thursday.

March and April saw an uptick in the share of young people signing up for private plans using the exchanges. These people, who are presumably healthier, are needed to balance out the medical costs of older, sicker consumers. About 35 percent of people who signed up throughout the open enrollment period were under 35 years old, including children, Obama said. Twenty-eight percent were between the ages of 18 and 34, according to a White House fact sheet.

The official six-month enrollment period ended March 31, but the federal government and most states accommodated people trying to complete applications in April amid a last-minute surge for subsidized private coverage and Medicaid benefits.

"This law is working," Obama said. "This law won't solve all the problems in our health care system. We know we've got more work to do, but we now know for a fact that repealing the Affordable Care Act would increase the deficit, raise premiums for millions of Americans, and take insurance away from millions more."

Obama urged Republicans, who remain almost universally opposed to the health care law, to move on. "I find it strange that the Republican position on this law is still stuck in the same place that is has always been. They still can't bring themselves to admit that the Affordable Care Act is working," he said.

With the congressional GOP continuing its nationwide anti-Obamacare campaign heading into this fall's congressional elections, Obama's exhortations are in vain. “The president may want to silence any further debate about Obamacare, but in doing so he betrays a lack of confidence in his own policies and scant regard for those most affected by the law,” Senate Minority Leader Mitch McConnell (R-Ky.) said in a written statement.

Nevertheless, the inaugural Obamacare sign-up period managed to exceed expectations in the end, despite the disastrous rollout of HealthCare.gov and troubles with state-run exchanges in Oregon, Maryland and elsewhere. The Congressional Budget Office originally predicted that 7 million people would enroll in private coverage via the exchanges, and later downgraded it to 6 million to reflect the troubled enrollment websites.

On Oct. 1, only 6 people were able to enroll using the federally run health insurance exchanges, and just 106,000 enrolled nationwide in all of October.

By contrast, 3.7 million people have signed up since March 1, reflecting a massive influx of new customers into the system as the March 31 deadline to get private coverage -- and avoid paying tax penalties for violating the law's individual mandate -- loomed. The Obama administration and most states permitted enrollments to continue into April for those who had started applications or whose applications were snarled by technical problems.

The share of young adults who signed up over the six-plus months of enrollment represents an increase from the first half of the sign-up period, when less than one-fourth of private plan customers were younger than 35. The White House originally hoped that around 40 percent of private insurance customers would be younger, and presumably healthier. The proportion of younger customers who signed up this year is nearly identical to the first year of Massachusetts' health care reform program in 2007, which served as a model for the Affordable Care Act.

The ratio of young to old -- being used as an imperfect proxy for the ratio of sick to healthy -- will help determine premiums for plans sold on the exchanges next year. Although health insurance companies like WellPoint already are speculating about rate hikes that exceed 10 percent, price increases on that scale may not be in the offing, and other insurers are more bullish on Obamacare.

A fellow at the Society of Actuaries who analyzes insurance, Dave Axene, predicted to USA Today that average premiums will increase 6 percent to 8.5 percent next year, compared to 7 percent to 10 percent in previous years. Rate increases will vary by state and locality, as well.

The Obama administration hasn't yet released a breakdown of the enrollment figures that would allow for a full analysis of the first year of sign-ups through the exchange marketplaces.

The 8 million figure touted by the president Thursday doesn't include what likely is millions of consumers who purchased plans directly from health insurance companies or through a private agent or broker.

Those policyholders are as important to the insurance systems in each state as those who signed up via an exchange because their medical costs will be pooled together, which will determine how much premiums will rise next year. According to a Gallup poll, this segment of the market disproportionately avoided the exchanges and obtained coverage directly from an insurer, a finding supported by previous data released by eHealth, an online insurance broker.

The latest numbers also don't fully account for the effects of the Affordable Care Act's expansion of Medicaid to more low-income people, which 26 states and the District of Columbia adopted. Between October and February, total enrollment in those programs increased by 3 million people, according to the White House. The number may increase, however, as federal and state authorities sort through incomplete and stalled enrollments in these programs.

Moreover, crucial information remains unavailable, such as what proportion of customers secured their coverage by paying premiums to their health insurance providers, and how many of the private plan and Medicaid enrollees previously were uninsured.

Polling, estimates from outside groups, and leading indicators from sources beyond the Obama administration hint at the answers to some of these vital questions.

Survey data from Gallup and others indicates the share of Americans without health insurance has declined since the exchanges opened for business in October. According to Gallup, the uninsured rate went down from 18 percent in the fourth quarter of last year to 15.6 percent in the first three months of 2014. The rate declined further in states that adopted the Medicaid expansion, created their own health insurance exchanges, or both, Gallup reported.

The Congressional Budget Office expects the number of uninsured to decline by 12 million during 2014, according to a report issued this week. The CBO maintains that 6 million, not 8 million, people will secure private coverage via a health insurance exchange, largely because some customers won't pay their first premium or will let coverage lapse during the year, and because some will cycle out of the exchanges into other forms of insurance. The CBO also reduced its estimates for the cost of the Affordable Care Act.

Most, but not all, of those who enrolled using a health insurance exchange seem to have paid premiums, with estimates cited by former Health and Human Services Secretary Kathleen Sebelius, health insurance companies and some state officials at 80 percent and higher. That share may rise in the coming weeks as bills become due for those who signed up near or after the March 31 deadline for coverage that begins May 1.

The enrollment numbers will continue to be fluid throughout the year. People who experience "qualifying life events," such as getting married or moving to a new state, can shop for health insurance on the exchanges year-round, and enrollment in Medicaid and the Children's Health Insurance Program can be done at any time. The next open enrollment period begins Nov. 15, 2014, and ends Feb. 15, 2015.