Being in a fairly rural part of the world, my local route only has one service, so waiting at the bus stop is enough to alert the driver to the fact that you wish to board. There was an occasion a month or two ago when I was waiting for a bus to Inverness (from Nairn), and both the 11 (via Airport) and 10 services were due at roughly the same time. I wanted to catch the more direct 10, but it was running a little late, and it was clear from the Stagecoach app that the 11 would get to the stop first. All I did was retreated back into the shelter a little, shook my head and made a "move on" gesture to the driver. This was clearly appreciated by the driver who gave me a wave to say thanks as he passed. When the bus I did wish to catch followed on a few minutes later, I put my arm out to make it clear that that was the bus I wished to board. I'm used to travelling on routes that have multiple services serving stops, and I have also experienced the sharp-tongues of drivers who have stopped for me when I simply wasn't paying enough attention.
I do think it's a wee bit unfair that you're singling out OAPs as the problem. They are human beings - please treat them with respect! In the case I mentioned above, I had access to the real-time information provided by the smartphone app. I was also near the junction where the two services merge, and it was obvious to me which service was which. I also knew that that particular run would have a coach allocated, but had I gone for the half-hourly inbetween service, anything could turn up. The bus stop had up-to-date timetables, but no real-time information display. If you have one service running behind the other when there's supposed to be a 10 minute gap between them, how do you know until it actually gets to the stop. How clear is the destination screen on the bus itself (does it actually work? I remember the days when paper destination strips were used) and are route numbers actually clear and distinguishable enough (11a/11b etc - the origin and destinations may be the same, but the actual route may differ)? Does your company make it clear that customers should stick their arms out to indicate which service they wish to board?
Even if all this information is present, and the company has done their best to educate the travelling public, you're always going to have people who are unfamiliar with the route, or simply stuck in their ways and won't be told either way. That's human nature, unfortunately. I've worked in retail and call centres for the last 20-odd years, and some people will always make your life awkward no matter how clearly you spell it out. It's one of the pitfalls of doing a job that involves serving Joe Public.