It's worth remembering that the average pensioner (I highlight that before anyone tells me they are not all rich) now has the highest disposable income of any age group, according to The Economist (their website is largely log-in only, so no linked source available). Whilst their absolute income may be lower, the disposable portion of it is, as a percentage, higher. As many others have commented, is it fair that 75-year-old Gladys can swan around on a day out for free, whilst 21-year-old Sally struggles to get to work because she has to pay full fare? (Or indeed that 40-year old Richard goes back to driving to work because he can't justify the additional cost of the bus no matter how 'green' he's trying to be?)
It would, no doubt, be too complicated to introduce, but a way forward that has popped up in the (non-transport related) office would be to put every "benefit" into a basket, and allow each pensioner or household to state, say, 2/3 of that basket as their preferences (so 3/5, 7/10, etc.), reviewable annually. As an example, a healthy person may chose to pay for the occasional prescription but have winter fuel allowance, free TV license and bus pass, whilst another with no public transport may prefer to drop the bus pass and pay if they go visiting but have free prescriptions. If pensioner A then requires regular medication, they would have to consider whether it would be more cost effective for them to pay for their TV license but have free prescriptions, or stay as they were.
It sounds harsh, but on the generic basis that those who are most in need of a particular service are least likely to shout about their need for fear of making it known, it would be an interesting exercise to see how appreciated, popular and useful each benefit really is.