U.N. Ambassador Andrew Young criticized South African Prime Minister John Vorster yesterday as "very much over the hill intellectually, and in every other kind of way."

Speaking to 300 employees of the Department of Health, Education and Welfare, Young said the U.N. arms embargo against South Africa was not intended to destroy that nation but to push it toward moderate leadership.

"There are people" in South Africa "with enough courage and confidence in their own society to give a different kind of leadership to that country," Young said.

He added: All we are trying to do in sanctions is say that . . . we are prepared to try to help the more creative, conscientious, moderate leadership to develop immediately.

"We think if it doesn't, they are fulfilling their own prophecies. When they talk about fighting to the death, their fears might come true."

An arms boycott is more effective than a total economic blockade, Young said, because it makes it uncomfortable for South Africans without forcing them to become completely independent.

Young said that as a civil rights worker in the South in the 1960's, he learned that "whenever you force people into a complete boycott, they found ways to cut back and resist." Young said he never knew a total boycott that was effective, nor a limited one that failed.