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.

Read: Clinton and Sanders clash on health care