What country does the word teenager originate from?

It originated from a mixture of Old Norse, Old English and Middle-American.

The exact evolution of the Word is as follows:

• Tjōn = Old Norse for "teen"
• Tēona = Old English for "teen"
• Tene = Middle English for "teen"
• Teen = Modern English
• Teen Aged = Middle-American phrase
• Teenager = Modern American

Ironically, around 1000 AD, the Word "teen" actually meant "suffering" or "grief". So someone who is teenage is at the "grief age".