I am reading this thread and it is making me smile. Some of the posts are realistic, others less so.
I totally disagree that manners is an age or generation problem, it is down to individuals. I am 50, have children in their mid twenties and my youngest is nearly 13. Nowadays I am in a wheelchair when out of the house.
During school holidays, it means that if my 12 year old DD and I go out, she will be pushing me in a wheelchair. Now that really does give you an eye-opener into manners. She politely says "Excuse me" if she is trying to get past people and will wait for others to come through a door before attempting to push me through. However, the number of people who totally ignore her is astounding. She is quite accepting of it, whereas I get far more wound up by the ignorance of some.
However, it is not consistent with any age group, just dependent on whether a person is paying attention to what is going on or whether they are too wrapped up in their own world to be aware of their surroundings. I have had people ask my DD if she needs any help and I have seen others, when we are waiting to view something, stand right in front of the wheelchair, so I can neither see or be moved. Either way, age does not seem to be a factor in the thoughtfulness of people.
On a different note, my 24 year old DD gets frustrated by the attitude, usually by the "elder" generation, that the younger generation are rude and lack manners. All four of my children have been brought up with manners and yet, however polite they are, people will still blame the "younger generation" for everything, when it is not the case at all.