I think you could also translate 'die Jugendlichen' as 'young people' here - not sure many people say 'young adults' that often. Anyone else agree?

Youngsters should be accepted too..

I wonder if all this has something to do with the genitive of adjectives??? If it does, pleas tell me how.

The correct answer you give here ('Young people") is not accepted when the question is reversed, where it must be "young adults."