I have noticed a decrease in manners in my lifetime. I have worked in schools and it is very different to the past. Rudeness was far more taboo a few decades ago, like many things. I have also noticed a large increase in antisocial behaviour and am frequently awoken by drunken neighbours screeching and banging. It makes me more anxious about leaving the house. However, while consideration of others is not increasing, the emphasis on shallow "social skills" and image, on the other hand, is increasing, particularly in the job market. It seems as though it should be the other way round.
But, even among the "rude" youth mentioned, there are still many unwritten social rules and codes to adhere to within their groups. Sometimes very strict. Cultural rules are an innate part of humanity. Those who cannot understand them, break them or who call out any BS are outcast from the social group. Among certain parts of youth culture, it is the rules that you have to be "rude", otherwise you are rejected.
I am among those who cannot pick up on most unwritten social rules in the groups of which I am supposedly a part. I know "please", "thank you" etc., but politeness and social codes extend to far more subtle areas than that and are pervasive. It has reached such a degree in the modern workplace, with so much emphasis on political and social maneuvering, image over substance, that even though I am highly skilled and qualified in the actual work, I am at a substantial disadvantage. It is seriously getting out of hand.