I believe Alex's comment is right. He just has a small typo that may be throwing you off: a portmanteau of teenager and dementia. Example: The boys were in serious trouble because their teenmentia left them unable to make good decisions.

is a work that I got in college, I'm studying translation and interpreting and I have to create a phrase with the words "To have teenmentia" it is an idiomatic phrase

It sounds like a portmanteau or teen + dementia => a reduction in mental capacity due to being a teenager. It sounds like a made-up word. What's the context?

Do you mean dementia? That is a disease of the mind that makes you forget things. I have never heard the word teenmentia.