WASHINGTON — People signing up for health insurance through the Affordable Care Act’s federal and state marketplaces tend to be older and potentially less healthy, officials said Monday, a demographic mix that could threaten the law’s economic underpinnings and cause premiums to rise in the future if the pattern persists.

Questions about the law’s financial viability are likely to become the next line of attack from its critics, as lawmakers gear up for the midterm elections this fall. Republicans quickly seized on the government’s progress report on Monday as evidence that the health insurance law would not work.

But administration officials expressed optimism that more young people would sign up in the months ahead, calling the latest enrollment numbers “solid, solid news” for the health care law. They said that interest in obtaining insurance through the marketplaces was increasing sharply across all age groups and that youth outreach efforts would become more aggressive as the March 31 open enrollment deadline approached.

“We’re pleased to see such a strong response and heavy demand,” said Kathleen Sebelius, the secretary of health and human services. “Among young adults, the momentum was particularly strong.”