"Lawmakers should not delay addressing the long-run financial challenges facing Social Security and Medicare," the trustees wrote in a "Message to the Public."
"If they take action sooner rather than later, more options and more time will be available to phase in changes so that the public has adequate time to prepare," the message said.
The Social Security trust fund, which is expected to provide assistance to more than 45 million people this year, will be unable to fulfill its obligations in 2033, three years earlier than projected last year, if trends continue, the trustees said in their annual assessment.
After that, incoming Social Security tax revenue "would be sufficient to pay only about three-quarters of scheduled benefits through 2086," the trustees said.
To pay the balance, Congress would have to either increase taxes or reduce benefits, analysts say.
The Medicare trust fund, which covers the part of the program that funds hospital care, will run out in 2024, the same estimate in last year's report, the trustees said.
At that point, incoming revenue from Medicare taxes will be enough to cover 87 percent of annual expenses. That share will decline to about 67 percent in 2045, "then rise slowly until it reaches 69 percent in 2086," the trustees said.
Medicare is expected to provide health insurance to more than 50 million elderly and disabled Americans this year.
Not since 1983 "have we come as close to the point of trust-fund depletion as we are right now," warned trustee Charles Blahous, a research fellow at Stanford University's Hoover Institution public-policy think tank. "Our window for dealing with [the shortfall] without substantially disruptive consequences is closing fairly rapidly."
"We must take steps to keep these programs whole for the future," senior trustee U.S. Treasury Secretary Timothy Geithner told reporters Monday in issuing the annual report.
The two programs together account for more than 35 percent of all federal spending.
In explaining changes in their Social Security projections, the trustees cited slower growth in workers' average earnings and continued unemployment in the slow recovery from the recession.
They lowered their earnings projection, primarily because of rising energy prices and "slower assumed growth in average hours worked per week after the economy has recovered."