What do young Americans want? Something different from what they’ve been getting from the president they voted for by such large margins.

Evidence comes in from various polls. Voters under 30, the Millennial generation, produced numbers for Barack Obama 13 percentage points above the national average in 2008 and 9 points above in 2012.

But in recent polls, Obama approval among those under 30 has been higher than the national average by only 1 percentage point ( Quinnipiac), 2 points ( ABC/Washington Post) and 3 points ( YouGov/Economist).

Those differences are statistically significant. And that’s politically significant, since a higher percentage of Millennials than of the general population are Hispanic or black.

The reasons for Millennials' decreased approval of Obama become clear from a Harvard Institute of Politics poll of 18- to 29-year-olds conducted in November.

That poll shows Obama’s job approval dipping to 41 percent, down from 52 percent in April 2013 and the lowest rating in any HIOP survey.

One reason for the decline is Obamacare. Only 38 percent approved of Obamacare (39 percent approved of “the Affordable Care Act”). Only 29 percent of those who were uninsured said they would definitely or probably enroll in the health insurance exchanges.

Those results were registered five to nine weeks after the Oct. 1 healthcare.gov rollout. Tech-savvy Millennials must have been astonished that government produced a website that didn't work.

They also perceived, accurately, that Obamacare health insurance would cost them a lot. The law passed by Democrats elected in large part with Millennial votes was designed to have people under 30 subsidize the insurance premiums of those older, less healthy people over 50.

The old tend to have significant net worth, and the young -- with credit card and student loan debt -- tend to owe more than they own. Evidently the Obama Democrats think it's progressive for the young to subsidize the working-age old.

That after all is the essence of Social Security, whose benefits some left-wing Democrats want to increase.

But Millennials, whose penchant for volunteering is admirably high, are not being simply selfish. The Harvard survey also finds that they tend to believe, by a 44-to-17-percent margin, that the quality of their health care will get worse under Obamacare.

That’s speculation, of course. But it suggests a healthy skepticism about the ability of a government, a government that lied about whether you could keep your insurance and your doctor and couldn’t construct a workable website, to produce a system that will improve service delivery.

That skepticism may owe something to young Americans’ experience with student loans. Some 57 percent of the Harvard study Millennials say that student loan debt is a major problem for young people. The responses don’t vary much by political party identification.

Once again, the Millennials have a point. The Obama administration did not initiate government student loans, but it continues to speak of them approvingly.

Yet it's obvious that the vast sums government-subsidized student loans have pumped into higher education over the last three decades have been largely captured by colleges and universities and transformed into administrative bloat.

Economics blogger Timothy Taylor notes that if you count prices in 1982-84 as 100, the average cost of all items in the consumer price index increased to 231 in September 2012. Energy, housing and transportation all increased about that much.

But college and tuition fees increased to 706 — seven times the level when the government started pumping money into higher ed. Medical care increased to more than 400.

Some things that young people buy increased much less — apparel (127), toys (53) and televisions (5, thanks to quality improvement).

But suddenly, in their early adult years, Millennials find themselves socked with the inflated costs of higher education and, thanks to Obamacare, those of older people’s health care.

In the meantime, in the Obama new normal economy, they aren’t finding jobs — and may be giving up on looking for them.

Labor force participation among those 55 and over has held steady since 2009. But labor force participation among those younger has been declining, as have earnings of college graduates.

The combination of higher education and health care costs and the new normal economy amount to what analyst Walter Russell Mead calls “the war on the young.”

No wonder they’re unhappy with the president who promised hope and change. Maybe they’re in the market for an alternative.