Green’s Dictionary of Slang

a young girl, usu. in very early teens, with a predilection for rock music and the boys who play it.

1967		W. Murray Sweet Ride 89: The Place was where it was happening [...] with a lot of boppers tripping and longhairs.
1971		L. Bangs in Psychotic Reactions (1988) 10: A crowd of blooming boppers, presumably cordoned off from their idols.
1978		S. King Stand (1990) 61: The junior high boppers will collect your records.
1999		Guardian Guide 4–10 Sept. 15: Adored by boppers far and wide.