I think it's a polite thing to do, but it in no way should be expected (unless there is some obvious reason the person will have difficulty standing - like they're blind, holding a baby, have a disability of some kind, are fragile etc).
There was a time I had back to back 12 hour+ days of mayhem at work, I was knackered, could barely walk to the bus, was falling asleep on the aisle seat. A guy who was about 50-60, who could quite capably stand by himself, walked up to me and yelled because I didn't offer him my seat. Others started having a go at me too (including some people who had seats themselves funnily enough), eventually one person who wanted to feel superior to me said "*I'll* give you my seat sir, not all of us are disrespectful" and a few people even clapped.
It was actually mental. What right did that guy who could quite capably stand have to embarrass me like that and expect a seat just because he is older? None.