To many Millennials, a "code of conduct" isn't something to help keep social interaction civil. It's actually a weapon that they use against those whom they dislike.
Seems to me that you're probably just finding the millennials more annoying because for whatever reason, their behavior has clashed with your outlook -- which is not to say anyone is right or wrong, just that there's an up-front conflict.
I have a feeling that the annoyance felt by the existing generations when they view the generations below them stems from those younger generations being not part of the established mindset of those of the older generation. David Bowie's song, "Changes," and the line, "And these children that you spit on, as they try to change their worlds, are immune to your consultations; they're quite aware of what they're going through." comes to mind. To each of us when we're the budding generation, the issues of our generation, our learned way of doing things, the problems and solutions that we and our peers have had to deal with, are all essentially established to us, and generally the generations above us, from our perspective, don't have these problems anymore. The younger generations that come up below us are not part of our established world-view, so sometimes that causes annoyance. Sometimes we get annoyed by their now experiencing the problems that we experienced and found our solutions to. Their habits, different than ours, annoy us as much as our habits annoyed the generations above us.