Congress has launched a major study of early retirement to see how it works, what it costs and who benefits from it.

Although the General Accounting Office does not propose legislation, its study would be the basis for any bill to: Let senior workers retire early. Save the jobs of younger employees who are most at risk during layoffs. Open up the promotion track for middle-age and younger workers.

The average federal employee is 42.1 years old and has 13.2 years of service. Most retire about age 61, but workers with 30 years of service can retire as early as age 55.

Although the typical federal worker is in mid-career, the prospect of layoffs casts a cloud on everybody's job.

Some federal officials worry that cuts -- triggered by the reduced Soviet threat or budget pressure -- could force layoffs of thousands of younger employees just before a large number of older employees -- hired in the late '60s and early '70s during the Vietnam War buildup -- are eligible to retire.

There are two early retirement plans pending in Congress. One, by Sen. William V. Roth Jr. (R-Del.) and Rep. Helen Delich Bentley (R-Md.), would offer a one-shot government-wide early-out to anyone within five years of retirement eligibility. That plan has been bottled up by the Democratic-controlled Congress. Federal unions oppose the Roth-Bentley bill unless agencies could replace workers who retired early.

Recently, Rep. Barbara Boxer (D-Calif.) proposed giving any employee who would retire an extra five years of age or service credit. Her plan, limited to Defense civilians, would mean a lifetime 10 percent pension bonus for workers who are already eligible to retire, as well as making retirement possible for anyone within five years of qualifying under regular rules.

Rep. William D. Ford (D-Mich.), chairman of the House Post Office and Civil Service Committee, proposed the study of early-outs in government and industry. In limited federal early-outs this year in agencies with 4,400 workers, about 16 percent of the eligible employees took early retirement. Despite years of offering limited early-outs to U.S. workers, the government knows little about how or if they work.

Among the things the GAO will consider: The cost/personnel impact differences of a broad early-out (like the Roth-Bentley plan), a more limited but generous early-out offer (like the Defense-only Boxer bill) and early-outs offered by automobile companies, International Business Machines Corp. and Pacific Gas & Electric. The assumed short-term benefits of early-outs vs. the projected longer-term savings from getting employees off the payroll sooner. Should government imitate private firms whose early-out offers are really one-time lump-sum buyout payments for those who waive future pension benefits? Should federal agencies make a special payment to the retirement system for each employee who leaves on early-out because that individual will be drawing pension benefits but no longer paying into the retirement system? Should management have the right to refuse early retirement to key workers?