Soviet party's youth group gets a new, younger leader

The head of the youth wing of the Soviet Communist Party has been replaced, a move long expected but conceivably deferred until the passing of Leonid Brezhnev.

One clear factor in the Dec. 6 replacement of Boris Pastukhov, 49, is his age. The new head of the youth organization, known as Komsomol, is 39-year-old Viktor Mishin. Mr. Mishin has spent the last several years in the Komsomol leadership, specializing in issues of ''working youth.''

Mr. Pastukhov becomes chief of the state publishing and book trade committee - a significant post in the Soviet ideological hierarchy, carrying ministerial rank within the government but affording much less input within the ruling party Politburo and Secretariat.

As early as March of this year, Soviet sources reported Mr. Pastukhov would be replaced as Komsomol chief. The sources, though not senior Soviet officials, would have been in a position to know of such a move.

Some foreign analysts speculate the change may have been deferred until the transition from the rule of party chief Leonid Brezhnev was more clearly resolved.