The flow of young professionals into Philadelphia has flattened, according to JLL Research, while apartment rents have started to soften in a number of big cities because of a glut of new construction geared toward urban newcomers who haven’t arrived. Apartment rents in San Francisco, Washington, Denver, Miami and New York are moderating or even declining from a year ago, according to Zillow.

“Certainly the softening of rents is one sign that they are not coming in at the pace that people thought they would,” said Diane Swonk, an independent economist in Chicago.

The debate is full of contours and caveats, but it really boils down to this: Are large numbers of millennials really so enamored with city living that they will age and raise families inside the urban core, or will many of them, like earlier generations, eventually head to the suburbs in search of bigger homes and better school districts?

Their choices — and it will be at least a few years before a definitive direction is clear — will have an impact on city budgets and gentrification fights. It could change the streetscape itself as businesses shift. It will affect billions of dollars’ worth of new apartments built on the premise that the flood of young people into cities would continue unabated.

It could also have a big impact on the American landscape more generally. For the past half-century, the trend toward suburbanization has continued with no real opposition. Even in the 1990s and 2000s, when urban areas were starting to turn around, subdivisions continued to expand. Have millennials ended that trend?

Here’s one thing we know: People get older. Another is that people’s tolerance for entry-level jobs and small urban apartments is highest when they are young adults. So while many things affect the increasing popularity of city living, including lower crime rates and a preference for walkable neighborhoods, one of the biggest factors is simply the number of people who are around 25.

Right now, that number is as high as it has been in decades. Another big driver of urban demand, immigration (both documented and undocumented), has been roughly constant since 2000. That number could change with policy, but given the current political climate, immigration seems unlikely to go up.

The last time the nation had a huge bubble of 25-year-olds was the late 1970s and early 1980s. Then, like now, an influx of people in their early 20s moved into cities. There were newspaper articles about young professionals gentrifying urban neighborhoods; “Changing San Francisco Is Foreseen as a Haven for Wealthy and Childless,” a New York Times headline said in 1981.

Each era has its economic challenges, and many millennials had the misfortune of entering adulthood during the Great Recession and its aftermath. The people at the tail end of Generation X, who came after the baby boomers, were born in the low-birth years of the late 1970s and entered adulthood during the late 1990s financial boom.

But the bulk of millennials came of age when jobs were scarce, Mr. Myers noted. They started moving into cities and apartments during years when rent was historically high. The number of young professionals was rising, adding to apartment demand, but supply was tight because new construction stalled during the recession. When city populations started swelling in the early and mid-2000s, it had less to do with the first of the millennials moving in than it did with lots of slightly older people not moving out, according to Kenneth Johnson, a demographer at the Carsey School of Public Policy at the University of New Hampshire.

The presence of so many young college graduates upended many American cities. You can see it in the hordes of techies congregating on weekends at Dolores Park in San Francisco and in the battles over gentrification. It’s behind new demands for rent control and the artists who live in cheap but unsafe warehouses.

In many cities, the high cost of living has galvanized a generation of young renters who have started to form a “YIMBY Party” that wants more apartment construction, and often pits younger renters against older homeowners.

Now, however, the pressure is starting to ease. Apartment developers have responded with a boom in new construction. In 2013, the number of people moving into and out of cities started to balance out for the first time since the recession, largely because those in their 30s and 40s resumed their march to the suburbs, according to Mr. Johnson’s research.

Some research also shows that, while millennials seem to prefer cities in their youth, the draw of the suburbs is still strong.

The counterargument is some version of “this time is different.” And millennials do seem to embrace cities more than their boomer parents did. They have shown up in greater concentrations, they bike more and drive less, and they have lingered in cities longer — possibly because they are still building a house down payment, delaying children longer or forgoing parenthood entirely.

For these reasons and others, Joe Cortright, director of the City Observatory, an urban think tank in Portland, Ore., is predicting that cities will continue to swell with young people coming in and older people staying longer. The decline in births between millennials and the generation after them — often called Generation Z — is more like a slight grade than a cliff, so even as millennials age there will be a new, though smaller, supply of 25-year-olds coming behind them, he said.

Still, if there is an economic lesson that the Great Recession has stamped upon many millennials, it’s that there is often a big difference between what you want and what you can afford. So while some 22-year-old millennials might have made a declaration that they would rather die than live in a suburb, their 35-year-old selves might feel differently, especially when they realize that suburban housing is often cheaper and suburban public schools are often better.

“You can have all the preferences you want, but you have to live somewhere and you have to have a budget,” said Svenja Gudell, chief economist at Zillow. “Those are the cold hard truths you have to live with.”

Source: Conor Dougherty, nytimes.com