Many Japanese people think that being a teenager means being anywhere from 10 to 19 years old. Actually, 10, 11, and 12 year-old children are NOT teenagers. If you don't know why, keep reading!
The word "teenager" comes from "teen" in the words thirteen, fourteen, fifteen, sixteen, seventeen, eighteen, and nineteen. That means a teenager's age can only be from 13 to 19 years old. "Ten," "eleven," and "twelve" do not have "teen" in them. It's that simple.
By the way, the photo above was taken almost 30 years ago when my friends and I were teenagers. That's me in the back!
* If you want strict correction of your English mistakes, take my lesson called Let's Fix your English!