What an interesting thread. So complicated: self-image, where one rides, how one rides, age, power, etc., etc. No wonder there are so many opinions. I still ride the Trek 5200 I bought in 2000, a 9 speed triple of course. I had a choice of the 5200 with Ultegra or the 5500 with Dura-Ace. I mention this because the 5200 cassette was a 12-25, whereas the Dura-Ace cassette was 11-23 and the Dura-Ace bike was significantly more money. So obviously, if you have more money you are stronger and will run a smaller cassette. Or is that just self-image and you pay for that image with more leg strain? Is that how this works?
Anyway as I aged I went from the big cog being 25 to 27 to 30. Then I went from the 30T granny to a 26T. That's still my gearing at 79. I've always done a lot of long climbs but mostly not more than 8% and I always turn at least a 78 cadence climbing. You're going to do multi-thousand foot climbs, you're going to want to keep the pedal effort down.
I also ride a tandem with my wife. It's a 2003 Speedster, also a 9 speed triple. In '15 when I was 70 and she was 66, we rode RAMROD on it running a 26X34 low gear. Climbs are only 7%. Tandems are not that popular on a 9000', 154 mile day ride That year there were 3 tandems and we were the second to come in, whoop-de-doo. We were killing 'em on that last 30 miles of gentle upwind downhill though.
When I was 74, we went to an 11-40 cassette which is still what we run. Of course the gaps are large, but switching between middle and big rings is a big help. Interestingly, the big ring has smaller gaps than the middle ring as we go up the cassette.