The Republican budget released on Tuesday is a daring one in many ways. Above all, it would replace the current Medicare with a system of private health insurance plans subsidized by the government. Whether you like or loathe that idea, it would undeniably reduce Medicare’s long-term funding gap — which is by far the biggest source of looming federal deficits.

Yet there is at least one big way in which the plan isn’t daring at all. It asks for a whole lot of sacrifice from everyone under the age of 55 and little from everyone 55 and over. Representative Paul Ryan, the Wisconsin Republican who wrote the plan, calls the budget deficit an “existential threat” to the United States. Then he absolves more than one-third of all adults from responsibility in dealing with that threat.

This decision doesn’t make him unique in Washington. There is nearly a bipartisan consensus that any cuts to Medicare and Social Security should spare the baby boomers and the elderly. And, certainly, retirees or people on the verge of retirement shouldn’t have their benefits changed radically. But the consensus, like Mr. Ryan’s plan, goes too far.

The reason is partly political. Older people vote in larger numbers than younger adults. Children, of course, can’t vote at all. But beyond politics, Washington’s age bias depends on a basic misunderstanding of the budget — namely, that older people have already paid for their Medicare benefits.