During the past decade, many U.S. cities have been transformed by young professionals of the millennial generation, with downtowns turning into bustling neighborhoods full of new apartments and pricey coffee bars.

But soon, cities may start running out of millennials.

Demographers, along with economists and real estate consultants, are starting to contemplate what urban cores will look like now that the generation — the United States’ largest — is cresting.

Millennials are generally considered to be those born between the early 1980s and late 1990s or early 2000s, and many in this generation are aging from their 20s into the more traditionally suburban child-raising years. There are some signs that the inflow of young professionals into cities has reached its peak, and that the outflow of mid-30s couples to the suburbs has resumed after stalling during the Great Recession.

Dowell Myers, a professor of demography and urban planning at the University of Southern California, recently published a paper that noted U.S. cities reached “peak millennial” in 2015. Over the next few years, he predicts, the growth in demand for urban living is likely to stall.

The flow of young professionals into Philadelphia has flattened, according to JLL Research, while apartment rents have started to soften in some 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 effect on city budgets and gentrification fights. It could change the streetscape 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 effect on the U.S. 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.