The role of teenagers in popular culture before the 90's was almost non-existent. They were not directly marketed to, nor where they prominent figures in 'grown-up' Hollywood features - meaning something that didn't involve a campy, or angst filled romp, usually involving a summer camp, or a popular hobby like roller skating, skateboarding, etc.
The advent of children's television stations, as opposed to just shows, were an invention of the mid-late 80's, and it just steamrolled from there. I graduated high school (in the USA) in 1989, and it was totally normal for adults to portray teenagers, especially in films that dealt with adult themes, such as sex, drugs, alcohol, etc. - for example, Jodie Foster's character as a prostitute in 'Taxi Driver' was not the norm.
The entire culture for those under 21 changed drastically once teens were seen as an audience with not only buying power, but also when technology advanced and they grew up with computers in school. It was simply becoming a younger, more progressive world, and as art does, it imitated life. Now, it's almost gone the other way - it's almost unheard of to see an actual person of appropriate age be in an American film - ie: in real life you would never find a police detective in their 20's, nor a high ranking government official under the age of 40.
The culture, mainly in North America, is very youth obsessed. But, again, with the advent of the internet, it has given an easier platform for pornography - which is typically a young person's domain for starring in - and, rampant consumerism, which preys upon people's normal aging as something that needs to be fixed by spending money on, the culture will continue to remain more youth-oriented, than not.