Hillary Clinton wants Americans to have access to Medicare starting at age 50. It’s an idea well worth considering, not least because so many Americans aged 50 to 64 who don’t have job-based health insurance struggle to afford the relatively high premiums they’re charged for private plans. And Medicare is a popular, battle-tested and relatively inexpensive insurance system, costing less per person than private insurance.

Such an expansion would have to be carefully designed, however, to make sure Medicare premiums for this age group accurately and transparently reflect the cost of coverage. Underpricing would undercut private insurers competing for the same customers, and also saddle taxpayers with the extra cost.

The most straightforward approach would be to offer people younger than 65 a price equal to the full cost of their coverage. That would probably be at least $7,600 for people in their early 60s — an estimate the Congressional Budget Office came up with in 2008. Today, the figure would be even higher, and it’s not clear how many people would want to pay it.

In contrast, the average second-most-generous Obamacare plan this year costs $10,911 for a 60-year-old. But the government subsidizes those premiums through a tax credit to people whose incomes are less than 400 percent of the poverty line. It would make sense to offer the same subsidies to people who would buy into Medicare.

If the price were well calculated, a Medicare option could offer needed competition in places where few companies sell insurance on state exchanges. That’s increasingly important: The number of counties where just one plan is available is projected to triple to more than 650 next year. And covering preventive care for people during the decade before they reach 65 could potentially lower Medicare spending on chronic diseases.

Selling Medicare policies on the exchanges would be an ambitious, yet not radical, way to expand insurance coverage. In other words, just the kind of incremental reform the health care system needs.

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