How housing that mixes young and old can improve the lives of both

Research has shown that older and younger adults need one another: Mixed-age interactions make seniors feel more purposeful, and young people benefit from their elders’ guidance and problem-solving skills. “They fit together like pieces of a jigsaw puzzle,” said Marc Freedman, chief executive of encore.org, a nonprofit group dedicated to uniting the generations.

But in practice, such closeness can be hard to come by. Many young adults flock to cities, while older people often isolate within the walls of 55-and-over communities. Parts of the country are as segregated by age as race, fewer people are having...