Halfway through the federal health care law’s enrollment period that started Oct. 1, more than a million Californians have signed up for some form of health insurance, according to figures released Tuesday by the state’s new insurance marketplace.

Covered California’s executive director, Peter Lee, said that by mid-January, 625,564 people had enrolled in an insurance plan through the exchange — an increase of almost 127,000 since Dec. 28, when the exchange last reported numbers to the federal government.

“That is more people … than those who signed up in the first two months of open enrollment,” Lee said.

In addition, about 584,000 Californians were determined to be eligible for coverage under the health care law’s expansion of Medi-Cal, the state’s health program for the poor. That figure does not include the 630,000 Californians who were moved over to Medi-Cal from the state’s Low Income Health Program on Jan. 1.

Three-quarters of the people who enrolled for an exchange plan that kicked in Jan. 1 have sent their first premiums to insurance companies. And 85 percent of the 625,564 are receiving subsidies that make their plans more affordable, Lee said.

Supporters of the health care law, officially the Affordable Care Act but commonly called “Obamacare,” were encouraged by the new numbers.

“As a result of the Affordable Care Act, over a million and a half Californians now have new financial assistance to afford health coverage,” said Anthony Wright, executive director of Health Access California, a statewide health care consumer advocacy coalition. “Most of these Californians are newly covered, or have coverage that is cheaper or better than what they had before.”

He also pointed out that the numbers don’t include the more than 400,000 Californians younger than age 26 who are allowed to stay on their parents’ policies.

Of the total who signed up for a plan on the state’s exchange through December, only 125,033 — or 25 percent — are 18- to 34-year-olds, often dubbed the “Young Invincibles.”

In contrast, 68.1 percent of those who enrolled in an exchange plan were between the ages of 35 to 64.

In California, health care experts say, the number of young invincibles needs to grow to 36 percent to help balance the exchange’s risk pool of younger, healthier enrollees with the number of older and sicker enrollees. Nationwide, if more of the younger group opt to pay the minimum $95 penalty instead, they could undermine the health care law by driving up rates on the new state exchanges.

Lee said Covered California is trying to target that group through an aggressive social-media campaign and coordinating more sign-up efforts on college campuses.

Patrick Johnston, president and CEO of the California Association of Health Plans, on Tuesday said that it’s premature to draw conclusions about the age mix on the exchange.

“We still have two more months for people to sign up for health coverage this year, and many younger people may wait until the deadline to sign up because they don’t need access to care right away,” Johnston said.

Ultimately, though, “it will be important to have a good mix of people with low health care needs to help balance out those with higher need, and age is only one indicator,” he added.

Open enrollment for plans on the exchange ends March 31; enrollment in Medi-Cal has no deadline.

Gerald Kominski, a health care economics professor at UCLA, acknowledged that older Californians are signing up at a faster rate than younger adults. But he said “we will not know” until later this year whether the lower-than-anticipated enrollment among young adults “will have a meaningful impact on next year’s premiums.”

Still, he said, the fact that so many people are selecting lower-priced plans on the exchange will prevent costs and use from skyrocketing “because those plans have relatively high deductibles, which have been shown in many studies” to drive down health care spending.

The low number of Latinos enrolling for health plans through Covered California is another concern.

While 18.7 percent of enrollees in the first three months identified themselves as Latino, 46 percent of all those eligible for subsidies on the exchange are Latino.

Lee said that means the exchange needs to work harder to assure that more Latinos apply.

Contact Tracy Seipel at 408-920-5343. Follow her at Twitter.com/taseipel.