Federal and postal workers could shelter up to 15 percent of their salaries in tax-deferred savings or investment plans as part of a new, three-tier retirement benefit program introduced yesterday by Sen. Ted Stevens (R-Alaska).

If approved by Congress and the White House, the program would be mandatory for the 300,000 U.S. workers hired since January 1984. Civil servants hired before that date could elect to remain under their current pension plan, which is outside of Social Security or come into the new system, which would base benefits on both civil service and Social Security.

The age for retiring with full benefits would be raised to 62 under the Stevens plan. Currently an employe can retire at age 55 with 30 years' service on about 53 percent of his or her final salary. Stevens' bill would still permit retirement at age 55 with 30 years, but with an annuity reduction of 2 percent for each year the individual is under age 62.

The Stevens bill would require the government to pay the full contribution for civil service retirement benefits. But employes would have to pay the full Social Security tax of 7.04 percent. Under the present civil service retirement program, employes contribute 7 percent of salary to the retirement fund but pay only the 1.8 percent Medicare tax.

Congress has until the end of this year to set up a new pension program for the post-1983 hires. If it doesn't agree on a new retirement plan for them, those employes will have to pay the full civil service and Social Security contribution starting next year.

Federal and postal unions have advised Stevens' staff that they are willing to consider his bill as a vehicle, but they want benefits improved, particularly for lower-paid employes who would be less likely to participate in the deferred savings plan. Under it, employes could contribute up to 10 percent of salary into the plan -- tax-free until they retired or drew out the money -- and the government would contribute 5 percent.