About eight years ago, Ms. Lewton appointed two friends who live in her building to oversee her future. Both have power of attorney and are executors of her will. She has filed important papers in clearly marked boxes, and also given explicit instructions on where she would like to be buried, what she wants engraved on her headstone and how the funeral should proceed. (She did not, however, prepay the event. “I’m superstitious,” she said.)

Both Ms. Lewton and Ms. Tint hope to stay in their apartments rather than move into a facility for the elderly. “I know a lot of people in my building,” said Ms. Tint. “It’s like an assisted living center or a dorm,” or, in modern parlance, a naturally occurring retirement community.

Others — both singles and couples without children — are considering cohousing situations with people of all ages.

“My fear in life is being put in a nursing home,” said Nancy Squires, 58, who owns an I.T. consulting firm in the Washington area. Ms. Squires, who is divorced with no children, has been saving for retirement since the 1980s. “I come from a family that’s thrifty,” she said. “I reuse paper towels. My friends laugh at me; I wipe my hands and put it on the counter until I come back. But I was taught that retirement was very important. My parents were not wealthy. They earned a nickel and saved a dime.”

She and her friends have bandied about the idea of moving in together. “I love the idea of a communal living arrangement with separate spaces and shared expenses,” she said. “Not like in the ’60s with LSD. More of a 21st-century model, like a large farm where someone has horses, another raises/trains dogs for others, some of the people might coordinate an organic garden, some might cook gourmet dinners a few times a month. It’s all about really living to your fullest without eating dinners alone — unless you want to, of course.”

Bill Strubbe, 58, a travel writer and painter living in San Francisco’s East Bay, plans to leave the country. In the fall, Mr. Strubbe, who has no children and is single, is relocating to a kibbutz outside Haifa, Israel, that he has been visiting since he was 20.

“I’ll be living among a community of people I have known all my adult life and has systems in place for care of the elderly,” he said. “Unlike the U.S.A., Israel has excellent health care for all its citizens, and that will take a big load off of my mind, knowing that I won’t be left flapping in the breeze if something happens to me.”