But that spread began narrowing after 2000, and it has closed with unprecedented speed during the slowdown. Since December 2006, the employment-to-population rate for young people has fallen by a dizzying 10 percentage points, from about 55 percent to just 45 percent. That decline, much sharper than in previous recessions, has reduced the share of employed young people to the lowest levels in 60 years.

By contrast, the employment-to-population rate for older Americans is slightly higher today (37.6 percent) than it was in December 2006 (37.4 percent). During the long slowdown, no other age group has increased its labor-force participation, notes Heidi Shierholz, an economist at the liberal Economic Policy Institute.

Together, these twin trends have produced an economy in which the oldest workers are now nearly as likely to be employed as the youngest. From January 1948 through September 2009, the labor-force-participation rate of older Americans came within 8 percentage points of the rate among younger people in only one month. Since October 2009, the difference between the two groups has been 8 percentage points or less in every month. One side can't start working; the other can't stop.

In some ways, the change reflects positive trends. Compared with the first decades after World War II, fewer young people are working partly because more of them are in school. And more seniors are working partly because rising education levels have allowed more of them to find satisfying careers they prefer to continue.

But most seniors extending their careers are doing so from necessity, because "the resources they were counting on to retire just aren't there," says John Rother, the policy director at AARP, the giant senior lobby. In the same way, the rapid recent decline in employment among young people hasn't been offset by a commensurate rise in college attendance.

These labor-market trends might be viewed as complementary or even as a benign opportunity for Americans to space out their work life over a different span--from 24 to 68, say, instead of 21 to 65. After all, as life expectancies lengthen, the U.S. can't afford its social-safety net without extending the retirement age. But that would require a systematic effort to help young people use their early 20s to expand their skills and experiences. That's not happening.

Instead, what economists call the idleness rate is rising: The share of Americans younger than 24 neither at work nor in school has steadily increased since 2007. That disconnection creates the risk of what Harvard University labor economist Lawrence Katz calls "a lost generation."

Faster overall job growth would be the best antidote to that threat. But the particular problems of young people demand more-targeted responses. Colleges and universities must see to it that more students don't just start their degrees but also complete them. As Segal says, those institutions must also accept "greater responsibility to ensure" that those graduates leave with skills employers need. Washington, meanwhile, should consider further expansion of AmeriCorps and other service opportunities for this civic-minded generation.

Above all, the class of 1967, which is growing reflexively hostile to government spending, needs to realize the interest it shares with the class of 2011: Unless today's young people ascend into well-paying jobs, it won't be possible to finance Social Security and Medicare for tomorrow's seniors.

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