Why did this book have such an impact on Americans of the 20th century and today?

John Lennon was murdered over 30 years after this book was released, and while the impact was pretty severe on him, that's not really an answer to this question. 

Catcher in the Rye broke new ground in literature because it broke so many conventions and challenged the conservative values of the late 1940's-1950s mainstream. It featured a young protagonist thinking and speaking like a typical teenager--which was pretty shocking to those who had never read such language in print. 

The book continues to have an impact today, and will continue to have an impact as long as there are young people who feel disenfranchised by society. People strongly relate to Holden Caufield because he recognizes a lot of negative things about society and desires to protect the helpless and innocent from them. His voice is the same voice as anyone who's ever been frustrated by his/her rather helpless role in the world, and he speaks strongly to those like him--on the cusp of adulthood and scared to death of entering the real nasty world.

How Did Teenagers React To The Book Catcher In The Rye?

The Catcher In The Rye had an impact when it came out, and still does now because the teenage population still exists and can relate to Holden. At the time period the book came out, teenagers were not as exposed to Holdens language, behavior, and the sexual references. Adults did not approve of this book for those very reasons and parents were once young and disillusioned themselves, but they’ve grown out of it, and they assume the rest of the world has grown with them.

Holden is trying to grasp at maturity. As a teenager this kind of behavior and attitude is still relevant, even if its dated. Our generation might be more technologically focused, but that does not change the simple feeling near all of us have: rebellion toward those older than us. Most teens are trying to grow up too fast just as Holden Caulfield did in Catcher In The Rye.

We all feel alienated by adults at one point or another, mainly just because they have more freedom than us. The ways that Holden reacts to these feelings may be old-fashioned and unheard of nowadays, but the essence of his character touches close to home to teenagers today.