A teen is a person aged 13 - 19, because of the the number ending in "teen", this age group is popularized because coincidentally it is also when the biggest amounts of changes occur in a person.

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Some people (what I can only speculate as children) ignorantly claim 18/19 year olds are NOT teens, and label them as adults yet fail to acknowledge teen isn't a title, it's an age categorization that's upto a government or a countries legal system to decide what they and what they do not proclaim as adults, some countries chose to label their citizens as adults as early as 15, some as late at 21, In accordance of Jewish law a boy become a man aged 13. WHEN you hit your next decade of life (20+), you're known as a 20-something, before that period, you're a TEEN.