My dad left me several boxes of 1954 Twin Cities Arsenal 30-06 ammo. It still shoots; I fired off a box last November. Properly stored it lasts a long time.
The problem with the old shells and bombs is that in the wild, the explosive actually becomes more unstable and dangerous over time.
Around 10 years ago, I bought a thousand rounds of 7.65 Argentine/Belgian Mauser ammo. It was headstamped FN 32 or 33. Every bit fired without a hitch.
On the other hand, I bought some Argentine .45 auto which was from the 50s. It was clean and bright but was about 90% bad. It had obviously been stored badly, probably over heated.
I got 1000 rounds of .303 British for my Lee Enfield when I bought it. Shot about 400 of it so far without misfire, and it was made in 1918.