I think that it is safe to say that the article is using the teen reference to drum up sympathy for Mr. Johnson and to give the impression that he shouldn’t be held responsible for his actions considering his “tender age.”
If you will notice he is still in high school, when he should have graduated last June. Then please take notice the blurb on his high school, which is obviously a continuation school or reform school.
I am willing to bet that this isn’t Mr. Johnson first encounter with the police department.
I completely agree. When is an eighteen year old a "teen", and when is he a "young adult"? Depends on who's writing the story.
Side note, one evening my then 15-year-old daughter referred to herself and her friends as "young adults". I informed her she would be a "young adult" when she was paying her own rent, buying her own food, and willing to take full responsibility for her actions. She reverted to "teenager" very quickly.