Moisture is the enemy of Ammo.
With age the primer will become less likely to ignite (what we call a dud). Some times you can get a poor primer to fire on a second or third try.
If moisture permeates the powder charge it is possible it will fissile (that is burn poorly) and you may end up with a bullet failing to exit the barrel (which is the very definition of a bad day).
But primed shells (empty) and bulk powder do deteriorate.
My dad had not reloaded for years, and we offered to clean up his reloading bench and incidentally load all his empty .348 Winchester hulls while we were at it. He had a couple of cans of old DuPont IMR 4350 that must have dated to around the time of the Cuban Missile Crisis (he and all his Army buddies bought out every gun shop in Atlanta while that was going on. Best excuse they ever had!)
The IMR had a crust on it and smelled very peculiar. We poured it out in small piles on a gravel driveway and ran quickmatch from pile to pile . . . it burned nicely and that was fun. I would never have used it.
We also snapped off all the primers that had been sitting in the empty brass, just because primers are cheap and misfires are a pain. We had a fun time at the local range, because we squared up this big old Winchester Model 71, let it off, and it went < pffft! > .
People kept moseying over and asking, "What in #@!! are you SHOOTING?" and laughed when we showed them.