For me, simply being polite.
But, if you give up your seat anyway, nobody would be impolite, would they?
It has certainly never happened to me!
If somebody gets on with a wheelchair or pushchair and I am in one of the folding seats, I don't just sit there and wait until they ask me to move. If I did, I could understand that they would be less polite about it, as I am making their life difficult for the sake of it.
Sadly that is something that is lacking in a very large majority these days. And I am not just singling out OAPs, but that is who we are talking about here.
You do seem to have a bit of a chip on your shoulder about old people.
From my experience, older people are far less likely to cause me any problems - they don't shout abuse at people on the bus or play their tinny mobile phone music at full volume.
But it's only a fairly small number of younger people that do that - If I said that 'a large majority' of young people were rude and incosiderate then that would be a horrible generalisation and completely untrue.
If a senior citizen behaves in a disrespectful manner then I believe that it is only right that they learn the lesson that respect and courtesy is a two-way street.
I (usually!) prefer to reply with courtesy rather than escalating things - we all have bad days, and you are more likely to have an effect with politeness.
But one thing to consider is that, sometimes, they are coping with constant pain - and just walking and standing is causing them severe problems.
(That can obviously be true of anybody, and isn't an excuse, but may help with understanding).
. My problem is with those that feel it's a given right.
While, yes, that's annoying - that is quite probably how they were brought up. And they would not have had any choice in the matter!