The struggle highlighted in Tuesday’s Post  about a young married couple deciding whether they can still afford the District is a concern echoed among city planners across the nation, particularly in cities that have rejuvenated themselves by attracting millennials (the suburbs, too, have also gotten in the game).  They fear being anchored by what Richard Florida, the prominent urban studies theorist, called a “barbell demography” —  a large young and affluent segment on one end; senior citizens on the other.

In cities with barbell demographies, young people leave as they get older; then get replaced with younger people who only stay for a couple years. This can have detrimental civic and social consequences. Some cities, such as Philadelphia, where a Pew report showed half of the city’s 20-34 plan to move somewhere else within the next decade, are in dire straights. In the District, where about 45 percent of those who left in 2012 were between ages 20 and 34, Planning Director Ellen McCarthy said her department sees this churn as a challenge they can conquer. But how?

Some wisdom comes from a national report published in April by the American Planning Association looking at ways to keep millennials and baby boomers from moving away.  In the millennial generation, 83 percent of those polled stated that housing and transportation costs would be the most important factors in decisions to stay or leave. Some suggestions from the report:

Cities and suburbs should invest in more walkable neighborhoods. The ideal location for millennials is a place in which a person could feel safe, raise children, enjoy a good meal and walk to work without breaking the bank.

Coupled with this must be more investment in a public transportation system that touches all crannies of the metropolitan area, allowing easier access to neighborhoods that might currently be out of reach.

McCarthy, the city’s planning director, added two more things that the District must do: lower crime and improve schools to make more neighborhoods desirable to young newcomers. That population has concentrated in some of the city’s hot spots — including Logan Circle, Gallery Place and Columbia Heights. Spreading the wealth could help lower rents. Art Rodgers, the city’s senior housing specialist, also points to the city’s first-time home purchase program, which provides up to $40,000 to help low- and moderate-income renters by a home.