I flew a couple of AT F and G motors today that were at least five or six years old, possibly a lot older. Both were in packages that HAD been sliced open, but re-taped shut (but NOT "sealed"). Both have been stored in a shed outdoors with no heating/cooling, and so have been exposed to way below freezing winters as well as VERY hot summers, although in a very low humidity atmosphere. Both lit and flew just fine. One ignited with the supplied copperhead. The other also came with a copperhead, but IT failed to fire so I used a homemade "starter" that is also a couple of years old, and stored in the same conditions - worked fine.
I have a lot of motors like this, and rarely have any issues.
AP reloads are pretty hardy, it would seem, and while it's never a bad idea to store them in stable environments it does not appear to be that big of and issue. I believe HUMIDITY is more of a concern than temperature.
In fact, I believe ALL the motors I've ever found to be hard to ignite were actually pretty new, and from packages freshly opened just before launching.
hmmmm.
s6