ASHINGTON, Nov. 12 — When Congress made it illegal 36 years ago for employers to discriminate on the basis of age, it was clear that older workers were protected from discriminatory treatment that favored those who were younger. The question in a Supreme Court argument on Wednesday was whether the Age Discrimination in Employment Act also meant the reverse: if Congress in addition meant to prohibit policies that favor older workers over younger ones.

The plain language of the statute suggests one interpretation, while the context in which the law was enacted in 1967 suggests the opposite. An employer shall not discriminate against someone "because of such individual's age," the law reads, without specifying "older age" or "younger age." The law's protection begins at age 40 and has no upper limit.

"The real issue here is whether this court should add an additional element" and treat the law as if it read "because of their older age," Mark W. Biggerman, a lawyer for a group of younger workers at General Dynamics plants in Ohio and Pennsylvania, told the court. He urged the court to avoid a judicial rewriting of the statute. "Congress's goal here was to make age a neutral factor in employment," Mr. Biggerman said.

In 1997, General Dynamics negotiated an agreement with the United Automobile Workers eliminating medical benefits that current employees would have received in retirement. The agreement allowed an exception, though, for employees who turned 50 by July 1 of that year. Some 200 workers between the ages of 40 and 50 filed suit on the ground that the new plan, which they were too young to benefit from, discriminated against them because of age.

The Federal District Court in Cleveland dismissed the suit, but the United States Court of Appeals for the Sixth Circuit, in Cincinnati, reinstated it on the ground that the law's protection ran in both directions. The Sixth Circuit was the first appeals court to interpret the law to permit a lawsuit of this kind.

The company appealed to the Supreme Court, warning that the consequences of this new interpretation would prove highly unsettling to management and labor alike. Indeed, General Dynamics came to the court with the support of the A.F.L.-C.I.O. and several individual labor unions, as well as the United States Chamber of Commerce and other groups representing management.

The age discrimination act "should not be stretched" to invalidate a plan aimed at protecting the interest of older workers, the company's lawyer, Donald B. Verrilli Jr., told the court.

"The truth of the matter," he said, "is that the word `age' is something of a chameleon that has different connotations depending on the context." In the current context, Mr. Verrilli said, if Congress had really meant to prohibit "youth discrimination," it was "exceedingly strange" to have chosen 40 as the age at which the law's protection begins. Rather, it is at age 40 that people begin to have a problem in being perceived as too old, he said.

Most of the justices appeared persuaded by Mr. Verrilli's argument, even Justice Antonin Scalia, perhaps the judiciary's best-known proponent of interpreting statutes according to their precise terms.

"So what happens is that a piece of legislation that everyone thought was meant to aid older workers, especially at the end of their working careers, ends up harming them," Justice Scalia said with evident disapproval to Mr. Biggerman, the plaintiffs' lawyer.

The Bush administration entered the case, General Dynamics Land Systems Inc. v. Cline, No. 02-1080, on the younger workers' side to defend a longstanding interpretation of the statute by the Equal Employment Opportunity Commission, the federal agency that administers it.

Interpreting the law to protect the young as well as the old "is perfectly consistent with the idea that stereotypes should play no role" in the workplace, Paul D. Clement, a deputy solicitor general, told the court.

The justices also issued their first formal decision of the term Wednesday morning, unanimously upholding the Social Security Administration's definition of disability.

In an opinion by Justice Scalia, the court held that as long as an applicant for disability benefits can still perform a previous job, the applicant will not be considered disabled even if the old job has become obsolete and is no longer available.

In this case, Barnhart v. Thomas, No. 02-763, the applicant, Pauline Thomas, was a 53-year-old elevator operator who sought disability benefits for heart and back problems after her job was eliminated. The justices overturned a ruling by the United States Court of Appeals for the Third Circuit, in Philadelphia, which had held that the ability to perform a previous job should not be disqualifying if the previous job no longer exists in the national economy.