If the opioid epidemic was the top issue plaguing American cities in the last five years, the most urgent problem of the next five is homelessness, a group of American mayors told reporters in D.C. this week.

Why it matters: Homelessness in the U.S. was on the decline after 2010, but it started to increase again in 2016 — and without moves to address the affordable housing crisis driving the issue, we can expect it to keep getting worse, experts say.

"This is the issue of our time," said Bryan Barnett, mayor of Rochester Hills, Michigan and president of the U.S. Conference of Mayors. "It's more people; it's more places .... Cities that haven't experienced it in the past are experiencing it now."

The backdrop: The primary driver of rising homelessness is the exploding cost of housing in coastal superstar cities like New York, Los Angeles, San Francisco and Seattle. L.A. alone saw a 26% jump in its homeless population between 2016 and 2017. Homelessness in New York increased 4% in that time.

• But the surge in home prices isn't limited to the most populous cities. It's a national trend, Elizabeth Bowen, a professor at the University of Buffalo, tells Axios.
• Between 1960 and 2017, median household income in the U.S. increased 29%. But in that same period, the median home price went up by 121%, according to a recent study by real estate company Clever.
• Now homelessness is not only getting worse in the cities were struggling with the problem, but it's also appearing in new places, such as Salt Lake City, Barnett said.

One effective solution would be to address the mental health crisis that underpins homelessness, said Michelle de la Isla, Topeka, Kansas' mayor who was homeless herself when she was younger. Those struggling with a mental illness on top of homelessness find it much more difficult to get back on their feet, she said.

• But "some cities and some politicians are finding it easier to say we're just going make the problem go away, and we're just gonna cover it up," says Bowen. "And one way to do that is to criminalize homelessness" and arrest people on drug or other charges.
• When mental health institutions are severely under-funded, "the largest resource for [homeless individuals] is the county jail," Kansas City, Missouri Mayor Quinton Lucas said at the event.

The bottom line: There are a host of ripple effects the country will have to contend with as homelessness increases, Bowen notes. One big impact will be an uptick in costs to the health care system as homelessness is linked to poor mental and physical health.

Go deeper: The zoning puzzle plaguing tech hubs

Nearly 10 million American kids live in low-opportunity neighborhoods, with limited access to good schools, parks and healthy food.

Why it matters: Simply being born in these pockets put these kids at a stark disadvantage. The neighborhoods in which children grow up shape many aspects of their adult lives, including how long they'll likely live, how healthy they'll be, and how much money they'll make.

The big picture: In a new report, researchers at Brandeis University used several factors — such as poverty rate, employment statistics and acres of green space — to assign opportunity scores (ranging from 1 to 100) to all 72,000 neighborhoods in the country.

• The hardest place to grow up is Bakersfield, California, where more than half of residents under 18 live in low-opportunity neighborhoods. The best is Madison, Wisconsin.
• Cities in the South generally have lower scores than those in the Northeast.

There are whopping disparities even within metro areas.

• Cities with the greatest opportunity gaps include Detroit, Baltimore and Philadelphia.
• In Detroit, there's a neighborhood with a score of 95, among the best in the country for kids, as well as one that scored 2. A kid born in the better neighborhood will likely earn much more and live up to seven years longer than another child born a few miles away.
• Opportunity is distributed far more equally in other cities, like New York, Los Angeles and Dallas.

There's also "an extremely clear and disturbing racial divide," says Dolores Acevedo-Garcia, the lead researcher and director of Brandeis' Institute for Child, Youth and Family Policy.

• Of the close to 10 million kids living in low opportunity neighborhoods, 4.5 million are Hispanic and 3.6 million are black.

What to watch: Some city leaders have turned Brandeis' child opportunity index into a guide to address inequities at home.

• Albany, New York, the city with one of the highest proportions of black children living in poverty, used the 2014 data to pinpoint which parks needed to be expanded and renovated.
• The Boston Medical Center built an app that tells residents in its most distressed neighborhoods where to find affordable, nutritious food.

The bottom line: "We've grown very complacent and grown used to seeing our metro areas so starkly divided," says Acevedo-Garcia. "But the fact that some kids live in these neighborhoods that have much, much, much worse conditions hurts all of us, as a country."

In addition to pointing to the urgency of homelessness, mayors at this week's roundtable — organized by The Hill — came to another consensus: rapidly aging populations are set to challenge U.S. cities.

The big picture: The median U.S. age jumped from 28 to 38 between 1970 and 2016, per CityLab. As cities get older, their mayors are tasked with creating policies and building infrastructure to adapt.

The mayors, who are in D.C. for the U.S. Conference of Mayors gathering this week, identified affordable housing and access to transportation as two urgent issues affecting seniors.

• Rochester Hills, Michigan is holding information sessions on autonomous vehicles for its seniors. Some older people might think "this is 'Jetsons'-type stuff that won't affect me," said Mayor Bryan Barnett. But when he tells seniors they're likely to live 10 years beyond driving age, and that AVs could improve quality of life, "they start to pay attention," he said.
• Kansas City, Missouri offers some public bus rides for free, and the bulk of riders are older adults, Mayor Lucas said.
• Fort Worth, Texas is infusing its city infrastructure with tech (such as cashless parking meters). To ease the transition for older residents who might be unfamiliar with the changes, the city organizes lessons at local libraries, said Mayor Betsy Price.

What to watch: Some cities are aging faster than others, CityLab reports. Communities in the Midwest and Appalachia are getting increasingly older as younger people move to the coasts, and managing aging populations will be an even more acute issue there.

Go deeper ... Read Axios' special report: The aging, childless future

A global trust crisis (Sara Fischer — Axios)

Reimagining the MBA (Patrick Thomas — WSJ)

The rise of direct-to-consumer orthodontia (Katherine Ellen Foley — Quartz)

The fall of the Forever 21 empire (Susan Berfield. Eliza Ronalds-Hannon, Lauren Coleman-Lochner — Bloomberg)

San Francisco tries to ease its housing crunch (Sarah Holder, Kriston Capps — CityLab)

The U.S. states with the highest rates of physical inactivity are clustered in the South, per a new map from the Centers for Disease Control and Prevention.

By the numbers: In Alabama, Arkansas, Kentucky, Louisiana, Mississippi, Oklahoma, Tennessee, 30% or more of adults called themselves physically inactive (defined by the CDC as "not participating in any leisure-time physical activities" like walking, running or biking in the last month).

• The least active was Puerto Rico, with 48% of adults self-reporting as physically inactive.
• The state with the lowest rate of inactivity was Colorado (17%).