In addition to those effects suggested above, there are a few factors that could affect this

1) College degrees might not be correlated with higher wages for young workers. I think you are right that people are spending more time in the classroom. In addition, people are spending more time in their early adulthood in a kind of extended adolescence.

2) College degree rates have gone up everywhere, so having a college degree confers less competitive advantage now than it used to.

In addition to those effects suggested above, there are a few factors that could affect this

1) College degrees might not be correlated with higher wages for young workers. I think you are right that people are spending more time in the classroom. In addition, people are spending more time in their early adulthood in a kind of extended adolescence.

2) College degree rates have gone up everywhere, so having a college degree confers less competitive advantage now than it used to.

Instead we are left with fairly meaningless statistic. Perhaps in the 1970s, the wage distribution was tighter around the mean — and perhaps now the 20-somethings are more divided between high-paid college grads and low-paid messengers. (As you can see, the median would be unaffected by such a shift.)

Come to think of it, since the 1970s, a huge chunk of the traditional middle class has left Brooklyn, Queens and The Bronx. These days, the city is largely comprised of four demographic zones:

A) “Manhattan”, including Brooklyn Heights, Park Slope, Williamsburg, etc.

B) Immigrant neighborhoods, such as Jackson Heights, Sunset Park, Ozone Park, Flushing, etc.

C) “Underclass” or “ghetto” or pick-your-euphemism neighborhoods, like East New York, Hunts Point, Far Rockaway, etc.

Though there are still some old-style middle class areas, they are vanishing.

So maybe there is something to my theory. Or, maybe not.

Instead we are left with fairly meaningless statistic. Perhaps in the 1970s, the wage distribution was tighter around the mean — and perhaps now the 20-somethings are more divided between high-paid college grads and low-paid messengers. (As you can see, the median would be unaffected by such a shift.)

Come to think of it, since the 1970s, a huge chunk of the traditional middle class has left Brooklyn, Queens and The Bronx. These days, the city is largely comprised of four demographic zones:

A) “Manhattan”, including Brooklyn Heights, Park Slope, Williamsburg, etc.

B) Immigrant neighborhoods, such as Jackson Heights, Sunset Park, Ozone Park, Flushing, etc.

C) “Underclass” or “ghetto” or pick-your-euphemism neighborhoods, like East New York, Hunts Point, Far Rockaway, etc.

Though there are still some old-style middle class areas, they are vanishing.

So maybe there is something to my theory. Or, maybe not.