[quote=Hank]
It literally says ager. Why is this a question? It lasts from the moment you enter your teens, AKA when you turn 10, until you turn 20, and are thus out of your teens and into your twenties. What you guys are talking about is , which starts at puberty (12-13) and ends at 18 (when you enter adulthood).
[/quote]

Look that up in the dictionary and tell me what it says. 

[quote=http://www.merriam-webster.com/dictionary/teenage]

: between 13 and 19 years old

: relating to people who are between 13 and 19 years old
[/quote]

[quote=http://dictionary.reference.com/browse/teenager]
teenager (ˈtiːnˌeɪdʒə)
 
— n
	a person between the ages of 13 and 19 inclusive [/quote]

[quote=http://en.wiktionary.org/wiki/teenager]Noun

teenager (plural teenagers)

 (countable) Literally, a person from thirteen to nineteen, an adolescent.
[/quote]

[quote=http://www.oxforddictionaries.com/us/definition/american_english/teenager]
teenager
Syllabification: teen·ag·er
Pronunciation: /ˈtēnˌājər
 
/
noun

 a person aged from 13 to 19 years.[/quote]

None of the definitions I found gave even remote mention to ten. Or twelve. It's commonly agreed that the term for teenager is applied to youths aged from 13 to 19. Not 10, not 12, but 13 to 19.