What happened to politeness?

Hello there!
Recently, I've realised how impolite people have become. We are raised to believe politeness costs nothing and gains everything, however as people are getting older they are starting to loose that idea. I decided to explore this theory.
I started with the youngsters. In working as an assistant performing arts teacher to children aged 6-8, I am often asked by them what they should be doing or if they can get a drink. I noticed that every single time any of the children asked me if they could do something they would always say please, and whatever my answer may be, they would say thank you, without fail. This showed me that their parents teachings had worked, and the idea of pleases and thank yous were engraved within their bright young minds already. Then I took a few steps up the generation ladder to children at the ages of 11/12, and all I can say is.. urgh. What happened in the 3 years between these ages I will never know, however in finding how rude children of this age were I was astonished. As I stood with my friends, I handed out cake bites to people who had asked politely to have one, then he came over. A 12 year old boy on the brink of puberty, with a voice like he had swallowed an dogs chew toy. He saw what I was doing and stuck his hand out to me and wiggled his fingers expectantly with a blank expression upon his oh so punch-able face. Let's take into consideration the fact that I had spoken to this child, what, twice in my life? Possibly 3 times? I looked at him, completely baffled, and asked him what he was doing, and (obviously) refused to give him any; to this he said 'wow, if only I had a friend who would give me cake bites'. His tone was dripping with sarcasm, as thick as treacle. I was a whisker away from saying 'If only you had a friend at all..' but I knew that that would be going completely against my plan of spreading politeness so I just ignored his remark and excused myself from the situation.
I'm still in disbelief of the fact that young people can be so rude and disrespectful, and I want to change this. I have taken it upon myself to will kindness and politeness on others, in the hope of improving their attitude.
It's a big task but i'm already seeing an improvement in people close to me.
Stay polite and stay beautiful.
D x