It's hard to explain, but before the 50s Americans didn't have the same concept of adolescence that they do now. Adolescence was considered to be the physical transformation of a child's body to that of an adult - and nobody gave very much thought to the emotional aspect of it.
American culture was once much less youth-oriented than it is now. There was no concept of teenagers as a distinct group with their own culture, language, problems and ideas - they were either viewed as large children or small adults, and past 18 they were expected to dress and act like full-fledged adults, plain and simple. Try to imagine a world where all the music, clothes, magazines and TV shows were either aimed at small children or the 35-50 age range. Now go to the library and look at some magazines from WWII.
Catcher in the Rye was the first American novel to realistically portray the emotional life of an American teenager, and presented it in an unflinching way that made Holden's struggles seem almost epic. The cliche was that every young person in the 50s who read it said "Wow, I thought I was the only one in the world who felt like this". Now, of course, virtually all pop music, most of the fashion industry, and a susbstantial part of the film and publishing industry exist soley to exploit and glorify the American Teen. So it's no wonder Salinger's book doesn't have the same impact it did in 1949. But in it's time it made a huge statement.