A teenager is literally a person who is anywhere from 13 to 19 years of age. In most western societies this is the age where an individual goes from being a child to becoming an adult. It is a period of rapid change both physically and emotionally. It covers the high school period into the early college years (for those who choose to go college after high school).
As a demographic, teenagers are preyed upon by advertisers because of their spending habits. As a group they seldom save money, seek an active social life, and oftentimes still have access to their parents' money.
Most Hollywood movies are geared to teenagers, and often take advantage of the anguish of that age to push an agenda of sex, violence, and rebellion.