"The term teenager refers to the teen years, as in the numbers that end in -teen. That's why the teenage years start at 13. So, yes, the teenage years includes 18 and 19 because they end in -teen." says google
google also says "Technically, they get the benefit of being both. However, the term “teenager” or “adolescence” is a social construct. In the legal system, only the terms adult or minor apply. Anyone under 18 (for most purposes) are legal minors while those 18+ are legal adults. So in the eyes of the law, 18 and 19 year old teens are adults."
"in the eyes of the law" but not in the eyes of most of SOCIETY therefore it is perfectly normal for an 18/19 year old to be seen as a teenager. most people still call 18 and 19 year olds teenagers. this is not a case of law but a case of a society that relies on social constructs. so therefore in the eyes of society and not just the law they can be teenagers. most people that i know call 18 year olds and 19 year olds teenagers, and i know a lot of people(from all over the world not just america, as i have lived in a lot of different countries). you cannot cite law as a valuable source in a societal ordeal, as some things that are ok in laws eyes are bad in societies eye, and some things that are wrong in the laws eyes are perfectly fine in societies eyes.