President Barack Obama repeatedly told Americans that under the Patient Protection and Affordable Care Act, “If you like your plan, you can keep it.”

But that wasn’t the only dubious selling point for the landmark 2010 health care overhaul, which passed without a single Republican vote — and with 34 Democratic House members voting against it. The massive, bewildering legislation also was pitched on the far-fetched basis that the federal and state insurance exchanges it created would attract enough relatively young — and thus, relatively healthy — Americans to help the bill live up to its “affordable” title.

However, as Tuesday’s New York Times reported: “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.”

The administration hailed Monday’s news that more than 2.1 million Americans had signed up for insurance through those exchanges from Oct. 1 through Dec. 28.

But that leaves a long way to go toward the White House’s goal of 7 million exchange policies sold by the end of 2014.

And these other numbers spell more trouble for Obamacare’s already-battered reputation: Only 24 percent of those who signed up by Dec. 28 were in the 18-34 age group.

That’s well below the 40 percent target for that crucial young-adult demographic set by the Department of Health and Human Services. Fifty-five percent of those who signed up were in the age 45-64 group. A third were in the 55-64 range.

You need not be an insurance expert to know that generally the older a group is, the higher its health-policy rates. And as Tuesday’s Wall Street Journal reported, insurers say the exchange-customer age breakdown released Monday makes “the pool older than they would need to sustain their coverage at current premiums.”

Yet HHS Secretary Kathleen Sebelius, seemingly oblivious to the statistical shortfall among 18-34 exchange customers, said Monday: “We’re pleased to see such a strong response and heavy demand. Among young adults, the momentum was particularly strong.”

That abysmal attempt at political spin would be more amusing if the fiscal stakes of the ongoing Obamacare debacle weren’t so immense.

Another revealing stat issued Monday: Nearly four-fifths of those who signed up for policies through the exchanges over the last three months qualified for federal subsidies to reduce their premiums.

In other words, we’re another step closer to “Medicare for all.”

And before assuming that’s good news, keep in mind that Medicare is headed for bottom-line oblivion without long-overdue reforms — including painful doses of benefit cuts and tax hikes.

As for the notion that the “surge” in exchange policy sales signals the start of an Obamacare comeback, consider this motivating factor reported in Tuesday’s Times:

“Many people who bought insurance on their own have received notices saying their policies were being canceled or discontinued because the policies did not comply with coverage requirements of the new health care law.”

In other words, if the Affordable Care Act doesn’t like your plan, you can’t keep it.

And clearly, lots of young Americans don’t like the Obamacare exchange insurance deal.