I would give up a seat for someone of any age or gender who appeared to need it more than I do.
The trouble is that we can't always see who needs it. I have arthritis in my knees, and sometimes I am in a lot of pain, but I don't use a stick, so I look fit and well (I am 60, so between old and young, I suppose). I have lost count of the dirty looks I've had on public transport from people who think I am sitting whilst someone less able is standing. I am paying for a seat, and shouldn't have to explain my situation to anyone else.
I have also found that it is older people rather than young ones (if we are generalising) who are very rude on public transport. Shutting (or opening) windows without asking people if they mind, putting bags on seats, pushing past queues to get a seat etc.
There is one older lady who gets on the bus near me. She has a walking frame, and needs to have a seat that will accommodate that (ie an outside seat or one near the gap meant for a wheelchair/buggy). Fine. But she arrives a second before the bus is due, when others have been waiting for ages, literally pushes through the waiting passengers so she is right at the front, and takes that space for herself.
IMO, whereas she has an absolute right to sit there if there is a seat available, and if someone is in an accessible seat they should move to a different one to let her have it, she does NOT have an automatic right to a seat if the bus is full, and nobody should have to wait for the next one in order to let her get on. If she is too late for a seat, she should wait for the next one (there is seating in the bus station), and either way, she should wait her turn to get on the bus in the first place. I have seen her ask (tell) a mum with a toddler and buggy to get off the bus so that she can sit down, which I think is outrageous: 'You'll have to get off - I need to sit here so there is room for my frame, and your buggy's in the way'. Another passenger managed to move the pram so that they could both use the space, but come on!
I realise that she is just one older (and rude) person, but I'm not sure that age is always relevant - it's about understanding that we don't always know who is more in need, and that there are young people who need to sit too.