So sorry to point this out to you, but that all changed in 1972, when they changed the status of adulthood from 21 to 18. I remember it well.
Doesn't matter. Eighteen is still the teen years. So is nineteen. It's just simple English.
You are seeking to make an argument that you can’t support. 18 is still a teenager and an adult.
Some teenagers under the age of 18 are tried as adults but they are still teenagers.
There is much more meaningful information in the article than the arrestee being called a teenager. I think the D.A. spouting inane comments is more important.