What is a teenager?

[Brain Damaged]

héhé, than i'm partly at teenager
15...

If you are 15, then you are 100% teenager.

[Bardoly]

I read in some magazines that Teen is a modern invented age between childhood and adult (Apparently, there were no teenagers during the Middle-Age for example)

Very true.  The term, "teenager" is less than 100 years old.  People were considered "young men" and ""young women" or "young adults".

I think before, there was childhood and adult. Once the child was ready, he became an adult with all the responsibilities.
Nowadays, we have the teenagers. They have more responsibilities than child, but do not have the power of the adults. Isn't this problematic?

Absolutely.  This is quite problematic.

[Starke Von Oben]

Many people assume that teenagers were first recognized as a group in 1950s America, but it was in fact the Nazis who first gave teenagers or "youth" a sense of having some signifigance within society.

He alone, who owns the youth, gains the future.
-Adolf Hitler

[Brain Damaged]

Teenagers are given far too much freedom these days.