Hey, kids. Uncle Sam has a new investment offer for you. Even if you have several decades of productive work ahead—and thus a long investing time horizon—the White House wants you to consider a retirement plan that will invest in nothing but U.S. government debt.

Any financial professional who advised a young investor to avoid stocks and corporate bonds—and everything else except Treasury bonds—would be sued for malpractice. But asset allocation is merely one of the problems with the new “myRA” fund rolling out from the Treasury...