Wiktionary
a. Not yet adult. n. One who is not yet an adult.
Usage examples of "preadult".
The youth assigned to drive their sled was two years younger than Taimi, still bearing the shaven skull with a scalplock that was the sign of a preadult among the Kommanz.
He had ambitions to restock its basement vivaria with a private collection of captured animals, prime amongst which would be the preadult hamadryad he had yet to catch.
WERE WAITING as I stepped through the door into the taverno: three of them, preadult Yavanni, roughly the size of Brahma bulls, looming over me from both sides of the entryway.
Now he was, unquestionably, doing the work that he had looked forward to all his adult, and much of his preadult, life: defending the United States from attack by armed enemies.