''Yesteryear,'' said the 9-year-old, learning a new word from an old radio program. ''When was yesteryear?''
Yesteryear was when we were 9 and listening to Westerns and serials on old-time radio, only nobody called it old-time radio then. Today, the term generally refers to family favorites of the '50s and early '60s. Newly issued audiotapes of those shows sell extremely well and are especially useful on car trips with children.
Among the intriguing radio characters from the golden age of radio now on tape are Mark Trail, a one-man Sierra Club; Sergeant Preston, who fights crime and lousy weather in the Yukon; insurance investigator Johnny Dollar, who taught a generation what an expense account was (not to mention what an insurance investigator was); and Bobby Benson, the boy who owned a ranch.
An excellent catalog is available from Carl Froelich Jr. Among the best: the three Lone Ranger sets; The Stan Freberg Show (No. 1), with its 1957 politically correct parody, ''Elderly Man River''; and Adventure Outdoors, an anthology, with commercials, of Sergeant Preston, Mark Trail, and others. The Sky King announcer spreading it on thick for Peter Pan peanut butter is Mike Wallace.