A few thoughts:
* Climb rates will depend on the average grade. It’s likely the duration that is a wall, not the vertical gain.
* It sounds like you’re not eating enough. A sudden drop off at the 90′-120′ mark usually indicates glycogen depletion due to lack of feeding prior to that.
* It could also be connected to pace. Perhaps you’re going too hard. That will make glycogen depletion even worse.
* You could also be unaccustomed to the duration. If most of your workouts are 90′-120′, then that’s what you’ve adapted to. You’ll need to go longer to get used to going longer.
* If recovery takes that long, then I suspect it’s a combination of all three, especially the lack of enough food and, perhaps, too high an intensity.
Lastly, yes, being 57 makes a big difference. Gains will be harder to make, and detraining will be faster.