means A person between roughly thirteen and nineteen years old, in the stretch of life between childhood and adulthood.

from A surprisingly young word for a young age: "teen" comes from the Old English suffix -tyne, the same root that gives us the numbers thirteen through nineteen — literally "ten and three," "ten and four," and so on. But the noun "teenager" itself only arrived in the 20th century, an American coinage that gained traction in the 1940s as marketers and magazines suddenly noticed there was a whole stage of life — and a whole spending demographic — sitting between child and grown-up that nobody had bothered to name.