i think it's kind of the opposite, the concept of an adult doesn't exist anymore. movies are for 13 year olds, books are "young adult" fiction for 30 year olds, music is teenage bedroom pop by a 35 year old woman
There was a time--not too long ago--when the concept of the "teenager" as a distinct culture (aka marketing category) didn't really exist, and I think we're pretty much there, again.