The Reagan administration fudged a little yesterday on the issue of a subminimum wage for younger workers, saying it still favored the idea in principle but was not prepared to endorse any specific bill before Congress.

There was even a little uncertainty about the principle. In testimony before the Senate subcommittee on labor standards yesterday, Labor Secretary Raymond J. Donovan said that, though he "would support a subminimum wage for youth if it were shown that it would help to solve this grave problem" of youth unemployment, he is "not certain the subminimum is a cure-all."

Donovan, speaking for the administration, added: "There are a number of considerations that must be carefully examined before the administration can endorse a specific legislative approach."

Studies must be done to consider "the effects of youth differentials on the jobs of adult wage earners," and on turnover among younger workers, since under some bills they must be paid the full minimum wage after a stated period; to determine the effect of a two-tiered wage system on minority groups and the poor, and to evaluate "the experience of other industrialized countries that have special wages for youth," Donovan said.

The three subminimum wage bills under consideration include one that would eliminate the minimum wage, currently $3.35 an hour, altogether for workers under 18. Another would slice 25 percent from the minimum for persons under 20, and the third would cut the wage rate by 15 percent for these persons.

The latter two proposals would require that affected wage earners receive the full minimum amount after six months employment.

The emotion-laden issue, one of a series of important labor questions scheduled for debate in the current session of Congress, yielded a standing-room-only crowd and a bountiful crop of rhetoric.

Sen. Don Nickles (R-Okla.), subcommittee chairman and staunch advocate of free enterprise, said the "opportunity wage" was needed to help remove "what has become a national burden of unparalleled magnitude -- youth unemployment."

But Sen. Edward M. Kennedy (D-Mass.), the ranking minority member on the Senate Labor and Human Resources Committee, warned that "supporters of the youth subminimum must meet a heavy burden of proof" for their claims that a lower youth wage would help more young people find jobs without hurting their elders.

Kennedy also said that any study of the minimum wage should include the possibility of raising it -- to as high as $3.75 an hour. That possibility is one reason some business groups have backed away from broaching any amendments in the minimum wage this year.

Robert E. Bradford, executive vice president of the National Restaurant Association, was one who seemed as concerned about keeping the lid on the current minimum wage as he was about the possibility of introducing a lower one. He spent the first half of his testimony arguing against any increase in the minimum, then turned to the subminimum idea.

"The youth differential would not erase all the negative employment effects of recent minimum wage increases," Bradford said. "It could not guarantee every teen-ager a job, yet we believe it has the potential to expand youth employment and allow more youth to build the work experience and background they will need to advance their adult careers," he said.