For Sara Zeff Geber, the “aha moment” came a few years ago as she listened to a friend recount all the tasks she was taking on to help her increasingly frail 91-year-old mother.

Ms. Geber, now 74, realized that there was no obvious person to turn to if she and her husband needed a hand as they grew older. “Who is going to do this for us?” she wondered. The disquieting answer: “No one.”

Solo aging — people growing older without reliable support from adult children or other relatives — has become increasingly common, largely because baby boomers and Gen Xers, by choice or circumstance, are childless at about twice the rate of previous generations. According to AARP, about a third of people 50 and older now live alone and don’t have children, are estranged from their children or can’t depend on them or other family members for help. Millions more who, like Ms. Geber, are married without children, will also eventually be on their own after a spouse dies.

These solo agers face many of the same planning issues as older adults with children — figuring out how they will manage their future care if their health falters, where they should live and how to make their money last. But their different circumstances often warrant different solutions.

“The lack of an easily identifiable default person to step up if you need help means solo agers have to approach retirement planning with an extra layer of intentionality and urgency,” said Rob Lyman, president of Johnson Lyman Wealth Advisors, a wealth management firm in Los Altos, Calif.

If you’re a solo ager or might be one day, experts recommend you begin thinking about your options as soon as possible, before a crisis hits. “You cannot plan for every eventuality,” said Ms. Geber, who now has a support system in place, and after years as a management consultant in Los Gatos, Calif., has made a second career as a solo aging consultant. “But you can ensure your most urgent needs are covered.”