most younger people are fitter than your average pensioner, so would expect them to give up a seat on the bus. I notice not only the younger ones doing it, but the middle aged too, those who are able bodied, and will let an elderly person not in good health struggle down the bus to find a seat. I will always offer mine, even if i am not in good nick myself. It is polite and how i was brought up.
Some elderly are grumpy i grant you, but i would rather see they are seated than have a fall on the bus.