It's an unfortunate fact that for some, ageing means not just ageing physically but also experiencing changes in personality. I disagree with those that are saying that a person who is rude on a bus, who happens to be old, was probably always rude even when they were younger.
I just don't think this is the case for all older people. A large number of people DO seem to display a decline in awareness of what is acceptable to say out loud in public. I remember when I was due to give birth any day to DS1 and I was in the supermarket huffing and puffing and looking at the displays of Christmas cards. The shelves were really in a mess, and as I was looking at the cards on one shelf, a packet of cards from the shelf below just fell off of its own accord because it had been balanced precariously anyway.
An old lady in a wheelchair tutted loudly and shouted "Well, pick it up then! Tsk....these youngsters! (I was 30!)" assuming I'd knocked it off the shelf and wasn't going to bother putting it back. Even though I hadn't knocked the packet off the shelf AND I was struggling to pick it up from the floor being the size of a whale, she embarrassed me enough to make me pick it up and put it back. Her daughter who was pushing the wheelchair was so embarrassed and said "Mum!" and mumbled a "sorry" at me and looked really sheepish and apologetic. I, of course, came home and moaned about her behaviour to DH, referring to her as "some horrible old woman".
These days, looking back on that incident, I find it hard to get annoyed. I'm pretty sure that in her younger days, she wouldn't have been so cantankerous - the way her daughter was so sheepish and nice suggested otherwise.
I'm also going through seeing my 75 year old father go through personality changes the past year or two. Not becoming rude and cantankerous, but very withdrawn and resigned to sitting in a corner doing nothing. I know he hasn't always been like that but I can imagine a teenager who doesn't know him meeting him and thinking "miserable old git."
I just think it's important to remember that while some people DO keep their personality with them when they get older, for many others, they simply behave quite differently from how they would have when they were younger. If that makes me ageist, then so be it.
It's just very hard to distinguish between those older people who are being genuinely rude and those who have lost their ability to realise what's socially acceptable.
A man (40 ish) in the supermarket looked at DS2 (aged 3) recently when he was being a bit whingey and shouted "Oh fuck off!" at him and walked off! As horrified as I was, luckily DS2 took no notice and I took it that the man had some kind of mental disorder, be it Tourette's or something else. I COULD have screamed abuse at him, or called for security to have a word with him as it was just near the entrance, but I just think it's important to consider possible reasons WHY people might say the things they say, and temper our response accordingly.
Very difficult not to say the first thing that comes into your head though, I agree!