IPA: /ˈtiːneɪdʒər/
KK: /ˈtinˌeɪdʒər/
A person who is between the ages of 13 and 19 years old, typically in the stage of development between childhood and adulthood.
The teenager spent the summer volunteering at the local animal shelter.
Teenager → The word is formed from "teen" (derived from the suffix of the numbers thirteen to nineteen, indicating age) and "-ager" (from Latin "aetas", meaning age). The term "teenager" refers to a person who is in the age range of thirteen to nineteen years old.
Think of the suffix 'teen' representing ages thirteen to nineteen, and remember that a 'teenager' is someone in that age group.