I would give up my seat for a pensioner, definitely.
On the Daily Mail website yesterday, there was an article about people being frightened of offering pregnant women seats, incase they were fat and not pregnant.
My husband is on crutches and has just started working. When he gets the train home, he said so many women have a bag on their seat next to them and he asks them if they can move it (politely). He said so many of them huff and puff and give him dirty looks cause they obviously think their bag needs a flipping seat (or they don't want someone sat next to them). He has to sit down because it's an hour and a half journey and that's a hell of a long time when you're on crutches.
I personally, wouldn't hesitate in offering my seat to someone on crutches. Sorry I went off on a rant lol.