Back in the mid 1990's I used a substantial quantity of Corrosive Kynoch 7mm Mauser ammunition that had a 1937 headstamp without a single misfire. On the other hand, during the same period, I bought a couple of boxes of 1960's vintage Norma 7.62 x 54 softpoint ammunition and nearly a third of them were duds. The chances are good that the Norma ammunition was stored for some period of time in an attic or truck where temperatures exceeded 100 degree fahrenheit.
A good rule of thumb is that if the room temperature is comfortable for you then it will be for your ammunition as well.
In 2008, I used some 22 rimfire cartridges from the 1960s and earlier including a hodge podge of BB caps, 22 longs and 22 shorts that I found in a cigar box in my Grandfather's basement workshop some time ago and they worked fine even after 40 plus years in a cool basement. Five or 6 years ago a friend of mine sold me more than 1400 rounds of corrosive 8 x 57 mm Mauser ammunition for about 45 dollars. As it happens, I did not have a rifle chambered for that cartridge but at a cost 3 cents a round, I was willing to go out and buy one. About half of it was Turkish and the other half Ecuadoran. The Turkish Mauser ammunition was very consistent and reliable with few if any misfires. The cartridges were headstamped with years 1941 and 1949, both used the 155 grain spitzer bullet. In July of 2003, I fired both lots over my chrongraph and the average muzzle velocity of the 1941 lot was 2849 fps and that of the 1949 lot was 2850 fps out of a Yugo model 48. This is standard muzzle velocity for that particular loading.
The Ecuadoran ammunition was a different story. About 10 percent of the cartridges misfired. Sometimes they would go off with a second primer strike but most of the time they did not. While it was a good way of detecting flinch but it was somewhat distracting as well. Storage in a non climate controlled building in a tropical climate is the likely culprit although a defect in the manufacture of the primer is possible as well.
While metallic cartridges will last quite a long time, paper shotshells with fiber wads tend to absorb and hold moisture from the atmosphere. Any paper shotgun shells more than a couple of decades old regardless how pristine they look will be unreliable at best.