I would hope that my 14 year old would always give up her seat for an elderly, disabled or pregnant passenger. I am not actually sure that she would though. Not because of lack of thought, but we don't use public transport that much and I doubt that she has ever seen it happen.
This article made me think that I explain to her that she must do so, and that it would not be inappropriate behaviour to offer! Will be going on tube tomorrow, so will start my lecture at about 1 o'clock ..........
BJ's idea sounds to me like common sense. And one that you would hope the parents would teach. But, for whatever reason, don't.