As mentioned, storage is a crucial factor. I have lived in my present house 39 years. (That’s not very common anymore, and being retired, we sure don’t need more space … but I digress).
Anyway, I have motors that have always been stored in our basement. Cool, and dry. In January 2020, I flew a 2-stage using a 50-year old B14-0 booster with date code December 1969!), staging to a 1991 dated B6-6. Perfect flight. So that B14-0 spent about 14 years in other storage locations. My memory is hazy about where I got them. I moved around a lot in the 70s, and did not launch rockets very often, but all my pre-1983 motors have not lived in temperature controlled storage.
I have more B14 motors and plan to use them, so maybe that 2020 launch success was pure luck. At least 95% of my inventory has always been in my basement.

My only motor problems have been with Q-Jet B6 motors. I bought a few packs of As and Bs when first released, and two have worked, two have failed. These were the motors that needed labels peeled to fit motor tubes. I have heard speculation that they were might have been overpressed during assembly. This could cause issues certainly, and might explain their increased diameter and requirement to peel labels to make them fit 18mm motor tubes.

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Lee Reep
NAR 55948

Projects: Semroc Saturn 1B, Ken Foss Designs Mini Satellite Interceptor
In the Paint Shop: Nothing! Too cold!
Launch-Ready: Farside-X, Maxi Honest John, Super Scamp