“I think the most likely scenario is that the full retirement age will be deferred,” Michalek says. “Today’s 18 year olds might start Social Security at age 70 or 72.”

Lawmakers have other options. In their most recent report, the trustees recommended a 2.62-point increase to the payroll tax rate, from 12.4 to 15.02 percent. Under current assumptions, such a move would ensure Social Security’s survival to the end of this century.

“I don’t know what the long-term changes will look like, but I don’t think they will be enormous,” Jeff Spivack, head of wealth planning at Janney Montgomery Scott in Philadelphia, says. “We’re more likely to see course corrections. Changes are inevitable, but large changes aren’t called for.”

Younger workers are the most pessimistic about Social Security. According to the recent Pew study, 43 percent of 18- to 29-year-olds and 47 percent of 30- to 49-year-olds don’t expect to receive any benefits. In contrast, 31 percent of 50- to 64-year-olds felt like they wouldn’t receive benefits from the program.

Tyler Landes, founder of Tandem Financial Guidance in Kansas City, Mo., says he doesn’t build Social Security into millennial clients’ retirement planning at all.

“At this age, there are so many variables, it’s more about just making sure their savings rate is adequate,” Landes says. “It’s dangerous for millennials to expect Social Security will exist in its current form, but we have a long enough time horizon that we don’t have to count on it.”

Despite the uncertainty, most advisors deliver Social Security advice as part of retirement planning.

“From our perspective, Social Security is a key piece of an overall retirement plan,” says DiCastri. “It’s one of the few pieces of guaranteed income clients get. With the decline of pensions, most people don’t have retirement income coming from an employer.”

Without Social Security, retirement planning would look radically different. Spivack suggests that in the absence of the program’s guaranteed income, demand would increase for financial products that guarantee a steady retirement cash flow.

“We would have to amend retirement planning,” Spivack says. “There are alternatives, like annuities with living benefits. Social Security is a forced savings program with a broad benefit. Without it, there would also be people with nothing for retirement.”