Before adult, after child.
At eighteen and nineteen, even if there's the word TEEN in the number, you're an adult, not teen.
Look at the definiton of adult, 18+.
It wouldn't make sense to be both an adult and teen.
Also, "adult" movies are always 18+.
You'll often hear "A TWENTY year old TEEN got caught in an accident..." in the news, but they just want to sound clever. (it actually fails, because no one looks clever by calling a twenty year old adult "teen")
This girl is a teen, she's 14.