Research has shown that older and younger adults need each other: 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, CEO 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 children, and people live by themselves in record numbers, including 27% of adults over 60.

"Older and younger people are two of the loneliest groups in society," Freedman said. "We have to be as creative in bringing them together as we've been in separating them out."

One solution is establishing residential communities that are designed to nurture these bonds.

"There's a trend toward intergenerational living," said Elin Zurbrigg, deputy director of Mi Casa, a D.C. nonprofit that provides mixed-age housing through its Genesis program, in collaboration with city officials. Demand may be rising because of the pandemic, which has exposed loneliness as a serious health issue and has prompted many Americans to move for fresh starts. I talked to experts and residents about the ways that mixed-age interaction supports well-being - and how to cultivate those relationships, regardless of where you live.

How mixed-age communities benefit their residents

- They cultivate purpose. Many people prefer to socialize with their same-age peers, but age matters less if individuals share a common purpose, said Cornell sociologist and author Karl Pillemer.

A shared purpose with neighbors is what Estelle Winicki, a 78-year-old retiree, always envisioned for herself, but finding that wasn't easy. In Boulder, Colo., she rarely crossed paths with neighbors. Retirement homes looked nice, but "everybody's old," she said. Five years ago, her therapist suggested Bridge Meadows, which operates two complexes of townhouses in Oregon that bring together seniors, former foster-care children and their adoptive parents. Residents are encouraged to spend time with their age opposites.

Winicki, who lives at Bridge Meadows in Portland, doesn't need persuasion. She starts many of her days helping her neighbors' children get ready for school. "It gives me such pleasure to see these kids grow with a strong foundation," she said. "They know they can rely on me, and I like helping."

- They provide mental health support. "The first thing you see among all the generations [at Bridge Meadows] is the sense of 'I belong' and 'I matter,' " said Derenda Schubert, Bridge Meadows' founder and a clinical psychologist. Such an environment allows mixed-age communities such as Bridge Meadows to provide safety nets that protect residents' mental health.

Kristina Fleming, 23, suffered from depression before moving to Genesis, a 27-unit building in Washington where seniors live next to young adults transitioning from foster care with their own kids. At previous residences, "people didn't care about my feelings," said Fleming, who has a 7-year-old. "We were just neighbors." At Genesis, the elders relate to her mental health challenges, she said. When she's feeling down, they ask what's wrong, and she confides in them. "We're all one - different ages, races, ethnicities."

Fleming, a college student, is an example of how mixed-age relationships can also boost self-esteem. Last year, she taught elderly residents to use tablets to photograph the community's garden. Enriching their lives during the pandemic made her feel helpful and content, a connection that has been shown in research.

- They offer professional advantages. In other communities, the generational glue is professional. PacArts, a mixed-age building in the San Pedro area of Los Angeles, provides affordable housing to artists. Luis Sanchez, a 53-year-old painter, said he can count on his neighbors whether he's having a rough patch with health - he's had two kidney transplants - or his work. An older neighbor has hired him repeatedly to assist with large painting projects. "I've learned a tremendous amount," Sanchez said. "She knows techniques and materials I would've never used."

Eva Kochikyan is a musicologist and teacher residing at Ace 121, a similar building in Los Angeles County. "It's a close community because we're all artists," she said. She grew up in Armenia, where residents socialized regardless of age, but after relocating to Los Angeles, she barely saw her neighbors. In moving to Ace 121, the 41-year-old re-created the experience of a big extended family.

"It's almost like having several grandparents," said Tim Carpenter, chief executive of EngAge, a nonprofit that provides these buildings with mixed-age programming such as exercise classes.

Kochikyan recalled her 4-year-old wandering into the building's communal art studio, sitting right next to an accomplished painter in his 70s and picking up a brush. "No lecturing, just working together," she said. "These connections happen naturally."

- They may keep older people active. Seniors may get more movement when inspired by the vigor of youth. "Older people might try to keep up with younger ones," said Thomas Cudjoe, a professor of geriatric medicine at Johns Hopkins University.

Kochikyan thought of a neighbor as an "old grandma" after watching her frown during a solo workout. Since then, though, the baby boomer has befriended a group of children who enjoy kicking her yoga ball with her. During these sessions, her intensity picks up and her face lights up, Kochikyan said, "like she drops 20 years off her age."

Research suggests this estimate isn't far off. Older adults spending time with kids saw large upticks in strength and physical activity in a clinical trial of volunteers at elementary schools in Baltimore, for example.

How mixed-age interactions can benefit the larger community

Intergenerational examples are increasing in type and number. But a few communities provide what residents see as the best of both worlds: same-age living, along with interactions with youngsters away from home. These interactions, too, can benefit both old and young. Here are a few examples.

- They can develop social purpose through volunteering. At Sun City, a large 55-and-over community in Georgetown, Texas, north of Austin, residents run scores of clubs that volunteer locally with children.

"Sun City is amazing," said Georgetown resident Jenny Phillips, who works for Sun City's homeowners association and considers its residents models of civic engagement for her two kids. She describes the sea of gray hair that cheers Georgetown High School's sports teams, and how "they're cleaning a parking lot after a community-wide garage sale raising scholarship money for underprivileged kids."

The Sun City example shows that retirement communities can, like Bridge Meadows and Genesis, unite the generations through a social purpose. Older people, especially retirees, often have the time it takes to develop genuine connections. "They won't form if you're in a hurry," said Freedman, author of "How to Live Forever." "It's like baking a souffle. If you rush, it collapses."

Some elders prefer how Sun City draws boundaries around the mixed-age socializing, so teenagers aren't blasting music next door. "In facilitating these connections," Cudjoe said, "we should be sensitive to concerns that younger people may be disruptive."

- They can help youngsters learn and seniors keep their brains sharp. Experience Corps, co-founded by Freedman and now run by AARP, offers another form of mixed-age interaction: older people tutoring kids in reading. In a study of the adult participants, scans showed enhanced brain volume in areas susceptible to dementia.

What to be careful of when mixing generations

Pillemer, the Cornell sociologist, ran a Legacy Project for older people to share their practical wisdom. One of his programs matches groups of kids to an elder with similar experiences - being an immigrant, for example. The kids design questions, conduct interviews and present what they learned to the broader community.

Some high schools and retirement homes are embracing this program. But, Pillemer warned, "if you throw older and younger people together without preparation, it can have negative effects." His research shows that training is critical. For example, older trainees can build resilience to ageist attitudes.

Winicki, the Portland retiree, said training is important, too. Recently, she promised her 12-year-old neighbor a fun outing if he could explain the high-tech dashboard of her new car. At first, he operated the controls at warp speed, expecting her to learn by watching. "Kids know the systems so well," she said, "they don't need their hands on the technology to understand." But he wasn't teaching another kid. "Let me do it!" she told him. He adapted his style. Then she guided the vehicle to the nearest ice cream parlor, where they enjoyed their reward.