The Supreme Court yesterday let stand a ruling that a test pilot who flew supersonic jets over heavily populated areas couldn't be grounded with the claim that, at age 52, he was too old.

The case involves a section of the Age Discrimination in Employment Act of 1967 that allows an employer to take age into account where it is a "bona fide occupational qualification reasonably necessary to the normal operations of the particular business . . ."

So far as McDonnell Douglas Corp. of St. Louis was concerned, that was reason enough to ground Phillip W. Houghton, a production test pilot who was taking F-4 Phantoms through 1 1/4 hours of tests and maneuvers to make sure the aircraft were ready for delivery to the Air Force.

He was taking off and landing at about 170 miles per hour, closing on a target at 1,000 mph, and attaining speeds of about 1,400 mph in level flight.

The company tried for several years to ground Houghton. Finally, in 1971, when he was 52 - and four years older than any other test pilot who had ever worked for McDonnell - the company ordered him to take a managerial post. He refused this and alternative proposals, McDonnell fired him in December, 1972.

Houghton sued in federal court. The Labor Department intervened in his behalf.

The judge ruled for McDonnell, holding that Houghton's age was a safety risk. The pilot acknowledged that he tended to become somewhat more fatigued than he had in his 30s, but said he considered himself physiologically to be in his 40s.

The Eight U.S. Circuit Court of Appeals reversed. In an opinion by the late Supreme Court Justice Tom C. Clark, then in retirement, the appellate court cited a "mountain of evidence" against McDonnell's position. The opinion noted that:

All nine McDonnell test pilots who had been killed on the job were in their 30s.

The evidence showing pilots in their 60s less able to perform adquately than pilots in their 20s was "negligible and wholly insufficient to support an inference of diminished ability among older pilots."

The accidents rate among professional pilots declines with age.

The English Circuit ordered the case sent back for a determination of whether Houghton - now 58 - still can perform as chief production test pilot. If he can, he must be reinstated. In any case, he's owed back pay for six years.

McDonnell termed the ruling "as alarming as it is unprecedented," one that requires it "to experiment with safety."

The Supreme Court took other actions: WATER A "MINERAL"?

The 1872 federal mining law allows private parties to discover and extract valuable minerals from public lands.

In the early 1960s, Charlestone Stone Products Co., Inc., acquired title under the law to sand and gravel mining claims in 450 acres of public land near Las Vegas.

But minerals considered "valuable" by the Interior Department weren't found. So in 1965M Interior filed a complaint charging that Charlestone's claims were invalid.

The company and the government wrestled the case through administrative and then judicial forums. Last May, the Ninth U.S. Circuit Court of Appeals, deciding an issue that hadn't been argued during the 12-year dispute, held that one of the claims was valid because Charlestone had discovered in it a valuable mineral water.

In a Supreme Court brief, Solicitor General Wade H. McCree said the ruling was "startling," "disregards all the precedents" and "would permit private acquisitions from the public lands without . . . commensurate public gain . . ."

McCree asked the court to review the decision. Yesterday, it agreed to do so. INNOCENCE

Wilbur McDonald, a Chicago machinist, served three years of prison sentence of 100 to 150 years for the murder of a woman named Agnes Lechmann. Then, in 1973, another man confessed. Authorities promptly released McDonald from the Illinois penitentiary at Joliet. Then Gov. Daniel Walker granted a full pardon "based on innocence."

Now for the catch. A state can be sued only if it waives immunity. For wrongful imprisonment such as McDonald sued 18 months ago for $15,000 per year in custody, or 8.21 a day. McDonald sued 18 months ago for $15,000 - covering a period beginning with his arrest, but the Illinois Court of Claims hasn't acted.

He also filed a federal suit claiming the Illinois limit on compensation deprived him of due process and equal protection of the law. he lost in the Fifth U.S. Circuit Court of Appeals. The Supreme Court let the ruling stand. PRESUMPTION OF INNOCENCE

At the end of the trial of Michael Taylor, 20, for second-degree robbery in Frankfort, Ky., last year, his lawyer asked the judge to instruct the jury that, under the Constitution, Taylor was presumed to be innocent. The judge refused. Taylor was convicted and is serving a five-year prison term. Yesterday, the Supreme Court agreed to review the case.