“Hey Nineteen” was about their mixing engineer (Elliot Scheiner) who went on a date with a young girl ~ 20 years younger than him.
As an aside, Elliot Scheiner went on to create the engineering of Acura’s high-end sound system.
Don't know much about the back stories of rock.I figured out,only fairly recently,that "And Your Bird Can Sing" was about Jagger and his "bird",Marianne and I read once that the "sweet,sweet Connie" in "We're An American Band" was a real rock groupie who did certain "favors" for rock groups.