I feel like the underlying question here is when you are to be considered an actual adult (not just "legally an adult"), not some semantic nonsense about the suffix "-teen".
The answer is when you start working and paying for your living expenses, i.e. taking responsibility for yourself. Some people do this with 18.
Some people go to college on a student loan and pass their spare time partying. These people are still not adults, despite being "legal adults".
Afaik, in Germany there is a regulation that when you commit a crime as a "young adult" (teenager), there is an evaluation if you should be judged as an adult or as a minor.