I always thought teenybopper was synonymous with teenager. Here's MW:

Main Entry: teeny·bop·per
Pronunciation: -"bä-p&r
Function: noun
Etymology: teeny teenager + -bopper, perhaps from 4bop
Date: 1966
1 : a teenage girl
2 : a young teenager who is enthusiastically devoted to pop music and to current fads

Can't say I ever thought it referred specifically to a girl, or the younger teenage years, but it feels right to add those qualifiers.