Some federal agencies have quietly cut the grade and pay levels of 90-day political appointees assigned them by the Carter administration and takeover assignments.

The cutbreaks were taken, insiders say, because so many of the new temporary appointees are young, with minimum work experience, and because they come from jobs that were relatively low-paying compared with their new government assignments.

Most of the 90-day appointees fall into the Political Schedule C category. That covers about 1,000 confidential or policy-maing aides in Grades 9 through 15. Grade 9 pays $14,097 to start and Grade 15 begins at $33,789.

Most of the lower-level workers are secretaries or confidential aides to political appointees. The upper-level workers on temporary assignment are in policy-making roles, or directly support top political appointees.

Several top goverment personnel officials said the cutbreaks in grade were made by readjusting duties of the jobs to "better conform" with the experience and expertise of people assigned by the White House or brought on board by new Cabinet officers or their subordinates.

There also has been some grade reduction - although not as drastic as in the Schedule C pool - in the Noncareer Executives (NEAs) who make up the 500 to 600 politically appointed "supergraders" in pay levels GS 16 through 18. GS 16 now pays $39,629 and Grade 18 has a flat ceiling of $47,500.

"These people (the 90-day appointees) are bright as hell and good," one personnel official said. "But they are very young to be in these grade levels, and their work experience isn't all that impressive."

Many of the 90-day appointees also came directly out of the Carter campaign organization where salaries were low, as they usually are on political campaign staffs.

"You might have a guy with a degree or two, and a lot of political experience, but his last - and best - job might have been at the $8,000 to $12,000 level," an official said. "We just couldn't justify rocketing them up to $25,000 or $30,000."

Another factor in the decision to cut the grades of the new temporary appointees, is higher government pay that went into effect in October for lower-level workers and salary increases for upper-level workers in February. Grades could be reduced but the salaries still remain high.

Some agencies are making a great effort to advise - and continually remind - the 90-day appointees that they are precisely that: Temporaries.

Officials doubt that many of the appointees - there are several hundred by now - will be made permanent. But they expect many will find lower-level jobs in the agencies through the career route, and some are working frantically to solidify themselves permanently in their "temporary" jobs.

"Some of these guys with real or imagined political connections think they are going to stay on," an official said. "But I'd bet most of them will go out the door when their 90 days are up."

Officials expect that the drop in the average grade for political appointees will be a short-lived phenomenon. "Those who get into the system permanently will find a way to work themselves and their grades up," an old-line bureaucrat said. "Then we'll see the grades start moving up again."