The plan to reorganize the Senate committee system suffered a sharp setback yesterday as the Senate agreed to keep alive the Select Committee on Aging instead of folding it into the new Human Resources Committee.

The move to retain the committee as a general overseer of all programs for the nation's 23 million elderly was sponsored by Sen. Frank Church (D-Idaho), chairman of the committee, who came to the floor armed with 51 co-sponsors for his motion.

Meanwhile, Sen. James B. Allen (D-Ala.) moved last night to tack an amendment disapproving the federal pay raise for Congress, high-level executives and the judiciary onto the committee reorganization resolution. Allen said that may be the only way he can get a Senate vote on the issue before the Feb. 20 deadline. Either chamber of Congress can veto the pay raise by Feb. 20. Earlier, Senate leaders had predicted that Allen's plan would lose and that they pay raise will go through, though Howard H. Baker Jr. (R-Tenn.) said the vote could be close.

A final vote on Allen's proposal was delayed temporarily last night by a parliamentary argument on another subject.

The retention of the Committee on Aging was a serious blow to the committee reorganization plan. It presaged moves by George McGoven (D-S.D.) to try to keep his Committee on Nutrition as a separate unit, and by Sen. Quentin N. Burdick (D-N.D.), acting chairman of the Post Office and Civil Service Committee, to restore that unit, which was to be blended into the Governmental Affairs Committee. The AFL-CIO is lobbying heavily to keep alive the Post Office Committee, which has jurisdiction over federal and postal workers, with federation President George Meany reportedly personally calling senators.

If these moves succeed, it would destroy much of the fabric of the reorganization plan whose sponsor, Adlai E. Stevenson (D-Ill.), originally invisioned streamlining operations and ending overlap by reducing the number of Senate committees from 31 to 15. The Rules Committee, in sending the measure to the floor, voted to retain the Joint Economic, Joint Internal Revenue, Veterans, Ethics and several other units, thus raising the number of surviving committees to 23; the vote to retain the Committee on Aging raises it to 24.

Stevenson at first sought to resist the Church amendment, arguing, "This is the beginning. Every organization representing special interests will seek to preserve their special committees."

However, faced with the massive support demonstrated by Church, who was backed by lobbying from labor unions and organizations of the elderly, Stevenson yielded and agreed to keep the Committee on Aging if Church reduced the number of members from 14 to 9 and made certain other changes. The deal was cut by voice vote and sealed on a 90-to-4 roll-call.

The Committee on Aging is strictly a study committee and can't report legislation. Church said it could look at programs all over, not just those whose legislation is handled by the Human Resources Committee. For example, the Finance Committee controls Social Security, Banking controls some housing for the elderly.

Survival of the Church committee, the Veterans' committee, the Small Business Committee, and if it happens, of the Nutrition and Post Office committees, will be the result in large measure of lobbying by organizations representing persons affecting, who believe they get better service and more response fro mthe special units many of whose members they help elect.

One lobbyist said the AFL-CIO postal and government employing maximum clout into an effort to keep Post Office alive, since it can influence pay and work conditions of postal and government employees.