(Adds more Obama comments)
WASHINGTON, April 17 (Reuters) - President Barack Obama delivered a vigorous defense of his signature healthcare law on Thursday, saying private insurance enrollment under it has reached 8 million people and faulting Republicans for failing to agree with him that "this thing is working."
The White House separately offered up statistics showing that 28 percent of those who signed up are between the ages of 18 and 34 years old.
Administration officials have previously talked about achieving a level of 38 percent of people in that age range, to give insurers a strong mix of healthier members whose premium payments help offset the cost of older, sicker policyholders.
"I've said before this law won't solve all the problems in our healthcare 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 told reporters at the White House.
"Which is why, as I've said before, I find it strange that the Republican position on this law is still stuck in the same place that it has always been. They still can't bring themselves to admit that the Affordable Care Act is working," Obama added.
Obama, who was on the defensive for months over the troubled rollout of the law starting last October, went on the offensive against congressional Republicans for their repeated efforts to repeal the law.
He said under the law, the share of Americans with insurance has grown, the growth of healthcare costs has fallen, hundreds of millions of Americans who already have insurance now have new benefits and protections and no one with a pre-existing health condition can be denied coverage.
"Those days are over. And this thing is working," Obama said.
He said the number of Americans who have signed up for private insurance in the marketplaces set up under the law, dubbed Obamacare, has grown to 8 million people. (Reporting by Will Dunham; Editing by Eric Beech)