Gee, this is difficult. I can only speak from personal experience, so please take what I say as such. Since you want concrete examples that's where I'll start.
Children's manners: I went to elementary school in the 60's, and although some people may think that kids were better behaved back then, I don't think they were any more polite than today's kids. Just looking at the surface, some kids in my classes in 2001-2003 behaved badly. But most of them kindly refrained from doing so when I asked them in a polite request. Examples, examples, of rude behavior:
laying legs on the desk/table during class.
talking aloud in a context not concerning studies
passing memo around during class
eating or drinking during class
boys pulling girls' hair
excercising irony, and hence disrupting instruction
The few instances where this simple solution did not work were usually family related cases. Unhappy situations at home or school were often the cause of rude behavior. On the whole, I would say that kids nowadays are much more responsive in communication. And I think without the strict, authoritarian upbringing that my generation are used to, the kids are more genuine and easier to communicate.
What I do miss is some basic manners that some parents fail to inform their children. One mother (my wife) would rather not teach the child (my son) the proper way to speak in a certain situation. I only had to let him know once what was proper, and that was enough. This example concerned how to properly say good-bye in Korean in the register of honorific. (=keigo ?h?? in Japanese?)
So as long as parents are willing to impart those basic manners, then the child will usually cooperate; all the better for him/her. If there is a slackening of manners, I think it is at least partially the parents' being irresponsibile in not caring to correct their child when it is really no big trouble for anyone.
EDIT: I thought I was the first to respond, but Duo beat me!
As for adults, I do find some adults misbehaving. I really hate it when I get asked or pressured to finish up and leave in a restaurant. I hate it when there is pressure (in the form of a frown) when people in a party each ordering a different dish. Funny thing is my companions are more than willing to co-operate even without pressure. I tend to be a bit more picky and avoid rude establishments as much as I can. I wonder why some business owners don't get it that rudeness is hurting their business? A harsh business owner usually gets paid back by losing customers: that should be obvious, but I guess not!