The nine justices of the Supreme Court can serve for life if they want to, subject only to a constitutional requirement of "good behavior."

Yesterday, the justices - average age almost 65 - heard arguments about the legality of a United Airlines pension plan that forces employees to retire solely because they reach age 60.

Justice Harry A. Blackmun, almsot 60, noted that "a large majority" of the justices (seven, to exact) are over 60, and then asked UAL counsel Arnold T. Aikens of Chicago whether that fact bothered him.

No, Aikens replied.

Chief Justice Warren E. Burger, who turned 70 two weeks ago, deftly ended the exchanged by recalling the [WORD ILLEGIBLE] rule of necessity, which holds that litigants are entitled the judgments even if it's awkward for judges to make them.

The litigant seeking a judgement in the UAL case is Harris S. McMann, an Alexandria man who was a pilot when he went to work for the company in 1944. He held a management job 20 years later when he joined UAL's voluntary retirement plan, signing application form that specifid the "normal retirement age" as 60.

McMann reached 60 - in fine health - in 1973. UAL then retired him, just as it has retired everyone in the plan at 60.

Meanwhile, Congress enacted the Age Discrimination in Employment Act. It took effect in 1968. This was 27 years after UAL adopted the retirement plan. It was also 4 1/2 years before McMann, his retirment imminent, notified the Labor Department that he intended to sue UAL.

His chief complaint is that the law's prohibition of age discrimination barred UAL from retiring him, solely because of age, before he reached 65. The Fourth U.S. Circuit Court of Appeals agreed.