means The years of a person's life from thirteen to nineteen, or the people living through them; also numbers in that same range.

from From the suffix '-teen' (Old English '-tēne', a variant of 'tīen', meaning 'ten') that gives us thirteen through nineteen — literally 'three-and-ten,' 'four-and-ten,' and so on. The plural 'teens' for those numbered years is recorded from the 17th century; its use for the people in them, the 'teenager,' is much younger, a 20th-century invention. So the word is, fittingly, built by adding to ten — a count that runs just past the point where childhood ends.