T. Coraghessen Boyle's "Greasy Lake" opens with the narrator stating that "courtesy and winning ways" are "out of style." The narrator goes on to state that "it was good to be bad," and people "cultivated a decadence like a taste." What this means is that people were no longer courteous, and it was considered appropriate to be bad. People, on top of being bad and without courtesy, were embracing the decline of a culture of morals.
In regards to what year this happened in reality, different people will have different answer to this. Some adolescents may not recognize that courtesy has disappeared. While the bad boy attitude has always been around, the lack of morality and courteous natures have not.
I would suggest that courtesy went out of style with chivalry. As feminists desired to open their own doors, pay for their own meals, and be a part of the work force, courteous behaviors have been on the decline. While not everyone will agree with me, I do believe that morality and courtesy began to decline as women desired equality and power.
(On a side note, I am sure that some people may misunderstand my answer and accuse me of anti-feminist ideologies. This is not my intent. Instead, I am only offering my educated answer regarding the point when courtesy began to decline. Although the story was written in 1985, courtsey and morality began to decline far before that.)