A unanimous Supreme Court, generally siding with workers in an important age-bias case, ruled yesterday that Trans World Airlines discriminated against its older pilots by not giving them other jobs when they reached their mandatory retirement age of 60.

But the court said TWA -- and presumably other employers -- could not be held liable for double damages unless it acted in "reckless disregard" of the requirements of the Age Discrimination in Employment Act.

The opinion, by Justice Lewis F. Powell Jr., took the middle road on a key dispute between the airline on the one hand, and the pilots, backed by the Reagan administration, on the other.

The pilots challenged a TWA policy that offered them other jobs when they reached age 60 but only if they bid for them. If another job were not available, they were forced to retire.

A pilot unable to fly for a reason besides age automatically would have been given another job without having to bid or wait for it, Powell said, calling TWA's policy on age "discriminatory on its face."

The pilots and the administration had also asked the court to interpret the law to say that employers were subject to double damages for age discrimination simply if they were aware of the antidiscrimination law and its provisions.

Powell rejected that argument, saying such a standard "would result in an award of double damages in almost every case."

TWA urged the court to allow double damages only if it were proven that a company intended to violate the law.

Powell rejected that standard as well, saying double damages could be awarded if employes were able to prove that a company had discriminated with "reckless disregard" of the antidiscrimination law. Powell, who is hospitalized and recuperating from prostate surgery, said that in this case, Trans World Airlines v. Thurston, TWA had acted in good faith and could not be held liable for double damages.

In another case yesterday, the court gave police extra latitude to arrest someone who they "reasonably suspect" has committed a crime.

The case began when police in Covington, Ky., acting on a "wanted" flyer issued by police in a nearby town, spotted and detained a suspect. They searched him and his car and found several guns.

The general rule laid down by the court is that police cannot make an arrest or a search without "probable cause" to believe that the suspect has committed a crime.

In 1968 the court granted a limited exception to that rule, saying that when police have a "reasonable suspicion" that a crime is in progress or about to be committed, they may briefly stop and frisk someone.

Yesterday, in a unanimous opinion written by Justice Sandra Day O'Connor, the court said that such stops and searches also may be made if police have reason to believe that someone was involved in a crime.

"The law-enforcement interests at stake in these circumstances outweigh the individual's interest to be free of a brief stop and detention," O'Connor wrote. The case is U.S. v. Hensley.

In other action, the court:

* Ruled 5 to 4 that a music publisher can continue to receive some royalties from record companies even after the author terminates the publisher's rights to the work. The court held that Mills Music Inc. could continue to receive royalties from licenses it issued to the song "Who's Sorry Now" from 1940 until the author's heirs took back the copyright-renewal rights in 1978. The case is Mills Music Inc. v. Snyder.

* Strengthened trademark rights, ruling 8 to 1 that a company may protect its registered trademark from infringement by another company even if the trademark is merely descriptive. The case is Park 'N Fly Inc. v. Dollar Park and Fly Inc.