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.