Why are old people so rude?

We're bombarded with information about the 'youth' of today being rude, ungrateful and selfish. But what about our ageing population? I live in a popular town where coach loads of (mainly older) people come to visit daily. The population of the town is predominantly white pensioners. So you get the picture, there are a lot of old people around (especially on market day). 
Why are old people so rude?

And, they're so rude. Not in a spitting chewing gum out, gobbing on the floor, shouting obscenities kind of way, but in a not saying thank you, walking slowly four abreast kind of way. I held open a door of a charity shop for five old people to walk in yesterday and not one of them said thank you. Yes, I was a bit of an arse and said 'thank you' sarcastically as each one passed me. 

But when did it become the norm to lose your manners as you get older? Maybe they never had manners in the first place? On Sunday, my son got hit in the head by an old man with a shopping basket who walked backwards into my him. When I pointed it out, the man just said 'Oh'. Not 'oh, sorry' or even 'I'll be more careful'.

Maybe I'm being overly harsh. Aren't we allowed to be fed up with the life by the time we're cashing our non-existent pensions in? Do we cut them some slack because they've lived longer than us?

I was a fairly horrible 'youth' and spat out my gum, hung around parks and shouted obscenities. I thought I was cool. I really wasn't and by the age of 15 I knew this. But surely when you get past a certain age you stop trying to impress people (I was about 23 when I stopped caring), so I hope the pensioners aren't trying to gain kudos by being rude.

I'd like to hope that I will still be able to say thank you to a kind person when I pass 65.