I purchased my 1st smartphone when I was 73 or 74 and immediately registered for the Krungsri mobile app and it was accepted. I replaced my phone 2 years ago with a new Samsung and transferred the app to my new phone. Again no problem. I was then 77.
I perceive from the T & Cs that once you have been accepted as a user of digital platforms by Krungsri, you are good for life. I cannot believe, for a moment that they will toss you under the bus on account of your age. And I'm 8 years ahead of you!
I believe that you are over thinking this, and both your experience with the bank and mine, seem to indicate thus.