The GOP’s brilliant generational weapon in the Medicare fight.

To senior citizens at town hall meetings angry or worried about their plan to convert Medicare to a private insurance scheme, Republicans have a simple answer: It’s not about you. You’ll be fine. This is for “the next generation.”

The next generation is everyone 55 or under, since the plan would not start for ten years and would affect only newly eligible seniors. The stated logic of the ten-year delay is that it takes time to put the system in place and that people need time to plan. But if “premium support” (a euphemism right up there with “enhanced interrogation”) were ever going to work, it could be implemented as quickly as the Affordable Care Act (four years) or Medicare’s prescription drug coverage (two years). Presumably, the delay is mostly a political kludge, intended to avoid a backlash from those now or soon to be dependent on Medicare by affecting only those young enough to be giving little thought to retirement health coverage.

But the line they chose is more than a gimmick: The 55-and-over cutoff marks a sharp and significant generational divide. Those over 55 will continue to benefit from one of the triumphs of social insurance in the Great Society, while the rest of us will be on our own, with a coupon for private health insurance. If you consider what it means to be 55 years old in 2011, you’ll see the significance of the line.

Today’s 55-year-old was born in 1956. That’s not generally considered a major break in the generations. It’s smack in the middle of the Baby Boom (the peak of the boom, in fact), with almost a decade to go before the first Gen-Xers were born, dreaming of Winona Ryder. But the difference between early and later Boomers, especially in their experience of the economy, is dramatic.

A baby born in 1956 would have graduated from high school in about 1974, from college in 1978 or so. Look at almost any historical chart of the American economy, and you see two sharp breaks in the 1970s. First, in 1974, household incomes, which had been rising since World War II, flattened. Real wages started to stagnate. The poverty rate stopped falling. Health insurance coverage stopped rising. Those trends have continued ever since.