My husband and I are half a decade older than you, but in pretty good shape (so far) with respect to backs and knees. My biggest problem is I am very small, so I have to keep my luggage size and weight in proportion. And I do strength training as part of my daily routine. I am able to manage Swiss train transfers just fine with a 22” roller bag and a small daypack on my back.
Here is what I see on the SBB schedule for your routes:
From Zermatt to Interlaken Ost, you have 2 changes, at Visp and Spiez. You will 11 minutes for each of them, which is plenty of time. Many people on the train will be doing the same transfer, especially at Spiez, so you can just move along with them (ask someone to confirm they are heading to Interlaken if you have doubts).
The Swiss train stations have underground passageways you use to cross the tracks to reach different tracks. In almost all cases, there are ramps as well as steps (or escalators at large stations). And an elevator, but that can be slow if there are lots of people with large luggage trying to use that. With roller bags you can use the ramps to avoid lifting your bags up and down steps. Some trains will have 2 steps up into the carriage, but I forget which kind of Swiss trains fall into that category.
At Interlaken Ost you change to the train to Lauterbrunnen, regardless whether you are going to Wengen or Mürren. You will come in on one of the tracks numbered 5-8, and must transfer to 2A for Lauterbrunnen. There are definitely ramps down to the underground corridor at this station; we used them several times last summer. The platform for Lauterbrunnen is designated 2A because they split the train at Zweilutscheinen and the back half (Platform 2B) goes to Grindelwald. You want to be in the right section of the train!
From Lauterbrunnen you can follow the underground passageway toward the Mürren cablecar, or stay at track level and walk that way. Lots of people will be doing the same. There are metal steps up to the boarding platform for the cablecar, but you hand your luggage off to the baggage transport guy before heading up the steps—-they will likely direct you to do this. Your luggage is transported in a bin attached to the cablecar, and then transferred to the little train that you take the rest of the way to Mürren. You pick it up at the station in Mürren.
If you are going to Wengen, you just go straight to the platform for the little red Wengen train which may be waiting there.
That covers Zermatt to Wengen/Murren. But if you are concerned about transfers, I will note that you are adding to your angst by heading to Chamonix. You can take a bus directly there from the Geneva airport. But to go from Chamonix to Zermatt, you will have 3 transfers, at Vallorcine (an easy, walk-across-the-platform one), Martigny, and Visp.
So that is the one I would eliminate if you are worried. It kind of duplicates Zermatt, anyway. If you want some time speaking French, you could head from the Geneva airport to one of the smaller cities on Lac Leman, like Vevey, or even Montreux, for your first few days. This adds some “lake time” to your trip, and the train ride from here to Zermatt will have only a single change on the way, at Visp.