There was a thread from jolly olde England that called them “youngsters.”
“Teenager” was always an artificial construct from the time it started to be used. There is a vast difference between thirteen years old, and nineteen tears old, in terms of behavior, capabilities, and ways of thinking.
The term was originally used to encompass those of typical age while in high school, then was spread upward and downward.
Juvenile, adolescent, youngster or whatever name used alternatively to lump this age group into one demographic are all inadequate to describe the very wide variations that exist. “initiated” or “uninitiated” is also a poor qualifier, as these life situations can happen well outside the age range under consideration.
Turning 18 does not automatically confer maturity, as turning 21 once did. Or did not. Growing older is mandatory. Growing up is not.