REP. CLAUDE PEPPER (D-Fla.) has decided to relinquish the chairmanship of the House Select Committee on Aging and will become chairman of the House Rules Committee. Born Sept. 8, 1900, Mr. Pepper is the oldest member of Congress, and his arrival at the Rules chairmanship may seem, to some, one of those elevations of an elderly patriarch a decade or two past his prime to a position he is no longer competent to hold and in which he will take positions the rationale for which he has long since forgotten.

In Mr. Pepper's case, this is almost the polar opposite of the truth. He has had a long congressional career -- but one interrupted by defeat and attended by controversy. He was elected to the Senate nearly 50 years ago, at the age of 36, and swiftly became known as one of Franklin D. Roosevelt's most ardent New Dealers. It did not hurt his career that he was also known as a spellbinding orator and as a "wingdinger" at helping Florida businessmen in post-New Deal Washington.

In time, Mr. Pepper's politics became precarious in the South, and he lost to George Smathers in the 1950 Democratic primary. That's the campaign in which the Smathers forces are supposed to have attacked the senator for practicing nepotism and for having a sister who was once a thespian. Had the election gone the other way, Mr. Pepper would probably have been chairman of the Senate Foreign Relations Committee from 1958 to 1981.

Claude Pepper returned to Congress in 1962, from a new and liberal district in Miami; it is a result of his seniority in the House that he is now in a position to become Rules chairman. Much has been made about whether he is giving up or gaining power by changing chairs. But neither position by itself confers much power. The Aging Committee has no responsibility for legislation, and before Mr. Pepper was chairman had no visible effect on the Republic. It was Mr. Pepper's advocacy of the bill to raise the mandatory retirement age and his stout defense of scheduled levels of Social Security benefit increases that made him a force to be reckoned with.

As for the Rules Committee, this was the body that once frustrated the will of the majority by refusing to schedule legislation. But since 1976, its majority has been loyal to Speaker O'Neill, and under Mr. Pepper as under his predecessor, Richard Bolling, Rules will be an ally of the leadership, not an independent power. This is one of the uncelebrated reforms in House procedure that have enabled this branch of Congress to act, as the Framers intended, expeditiously and with sensitivity to the will of the voters. Senior power, in the person of Claude Pepper, is no longer the product of an automatic seniority system, but of the talent, will and tenacity of the man.