American cities are becoming more and more unfriendly to families, and new parents are fleeing for the exurbs, where housing is more affordable and schools are better.

Why it matters: In a trend that is building its own momentum, cities are increasingly dominated by wealthy, childless residents. In the future, shifting local priorities could write kids out of urban life for good.

As the chart above shows, the share of young people under 20 years old in nearly every big city in the country has fallen over the last four decades. Zooming in, a few percentage points lost here and there seem minor. But taken together, they signal a major demographic shift in urban America.

"You're seeing [declining birth rates] in the most extreme form in cities. It's a window into a larger demographic trend where kids are few and far between."

— Stephen Mihm, economic historian at the University of Georgia

Experts chalk up the exodus of families to a number of concurrent trends:

"There's no doubt that a cluster of extraordinarily successful companies and the massive wealth that they've created has impacted housing prices," says Karen Harris, managing director of macro trends at Bain & Company. This is especially apparent in tech and finance hubs like San Francisco, New York and Boston.

that a cluster of extraordinarily successful companies and the massive wealth that they've created has impacted housing prices," says Karen Harris, managing director of macro trends at Bain & Company. This is especially apparent in tech and finance hubs like San Francisco, New York and Boston. "The high cost of living makes it hard for young parents to pay for the housing and living expenses of young children of school age so they move elsewhere," Terry Clark, a sociologist at the University of Chicago, tells Axios.

The result: Cities are barbells, with young, affluent and single people on one end and wealthy empty-nesters on the other, says Richard Florida, a University of Toronto urban theorist. Urban populations are constantly rotating as families move out and make way for newly-minted graduates who have disposable income to spend in bars and shops — and drive gentrification.

Even immigrants, who used to populate cities, are moving straight to suburbs where homes are cheaper and schools are better, Florida says.

The big picture: "Historically, cities have a hard time surviving, and much less thriving, if they must constantly replenish their populations from outside," Mihm tells Axios.

Big, vibrant hubs like San Francisco, Chicago or New York might not have difficulty luring scores of young people every year.

like San Francisco, Chicago or New York might not have difficulty luring scores of young people every year. But smaller cities like Hartford, Cleveland, Detroit and Rochester, which are too expensive for families and have a hard time attracting young talent suffer.

like Hartford, Cleveland, Detroit and Rochester, which are too expensive for families and have a hard time attracting young talent suffer. These four cities have seen the sharpest declines in kid population in recent years.

And if the most visible, most successful residents of cities are rich, single, young people, "schools, playgrounds and other amenities may not be funded with quite the same enthusiasm," says Mihm.

The other side: The kids haven't disappeared completely, but the families that do stay in cities are typically those that have the money to buy large homes and pay for private schools. "It's not that there aren't children in cities, it's that they're rich," says Harris."In fact, we've seen real renewals of cities with parks and museums and green spaces, but for the rich."