yes, I do see that sort of thing a lot too, I use public transport daily for my commute, most people are nice, the tutters are actually the rude exception.
I've seen "tutters" walk past available seats near the bus door and stand glaring over a teenager tutting about then not hopping up, when why should they? there were available seats a shorter distance from the door!
I tend to work on the assumption that people who chose to stand when there are available seats want to stand (off next stop maybe, and some people are more comfortable standing), so I don't get up for them, I also work on the assumption that if someone isn't getting up when there are no spare seats, they probably have a reason to.
Actual disabilities trump being in an age range where disabilites are more likely. Over 65s might really be less able to stand, and they and under 65s who are disabled have priority IMO over people who just happened to be over 65 but are otherwise fit. 2nd priority = all over 65s, 3rd priority = small children who can't balance well sanding.
However, disabled doesn't = wheelchair or crutches. Most are invisible.
An over 65 who can walk past available seats to make a point of tutting at a teen (who for all they know might actually need to sit near the door) is just an arse. It happens a lot.