Bad manners and ignorance are my main bugbear. Although to be fair I don’t think that it in any way is an age thing, I’ve encountered some incredibly polite and well mannered young people and by the same token some pig ignorant older people too. I genuinely think that they have no concept of the effect of their ignorance and the vexatious effect it has.
Just yesterday I walked out of a door and saw I guy approaching so obviously I held the door for him to enter. Not only did he not thank me he also never even glanced up from his phone screen, well not until my enough is enough moment when I quickly reached around, grabbed his hood and pulled him back through the door whilst saying forcefully to him “you’re ■■■■ ing welcome”.
At this point it could have ended badly for me as he was half my age, but luckily for me he just stood there bewildered asking me what he’d done wrong. So in his case there was no malice from his side, just simply brought up badly I’d say.