you mean 25 shells right? Or are you buying slug/buck shot. Most bird shot comes in boxes of 25

Chemical degradation of the powder and the primer will happen no matter what condition it is stored in. It may take longer to decompose in a more controlled environment, but the process is never halted, unless you are willing to go to extremes.

Brass will go brittle over time. Copper jackets will corrode. This can be mitigated some by applying protective coatings (as with tuff cloth). I had a box of 35 remington that was pushing 30 years old and had been kept in a relative safe environment most of that time. Upon shooting it, many of the cases exhibited stress cracks, particularly where the head meets the body. After 3 or so of those failures, I uncorked what was left of the box and deactivated the primers and tossed the lot. Generally, I don't like to keep ammo beyond about 20 years under any conditions, and shouldn't have to for now.

This is actually somewhat related to my line of work. The bottom line is, there is no simple answer because it all depends. If you take plastic shotgun shells, what are they actually made of? Some plastics (eg. polythene, PP) are relatively stable. Others like PVC or polyurethane deteriorate rapidly even under ideal circumstances. The decay cannot be prevented and is irreversible. While plastic objects degrade, they also tend to release a significant amount of gases. These are frequently corrosive (PVC is a good example) and will affect any other objects in the vicinity.

Even something as innocent as storing ammo in a wooden crate might be a bad idea. Wood, especially if not sealed, releases acetic acid. That means there will be a specific (slightly acidic) microclimate inside a tightly closed wooden crate. This may well corrode primers, bullets and brass cases and increase the rate of decay for many plastic shells. Most kinds of paper and cardboard are also acidic.

The damage is generally slight but cumulative. It really depends on other factors, mainly the room temperature and relative humidity (RH). As a rule of thumb, it's best not to keep plastic and metal objects stored inside the same tightly closed container. Anything plastic is better placed on open, ventilated racks, only lightly covered by something breathable to avoid dust accumulation. RH should definitely not exceed 60%. 50% or less would be a better (and usually still realistic) goal.

Folks living a dry climate may not need to bother with such safety precautions. In a very high humidity environment though you could store centerfire or rimfire ammo in a tightly closed metal box, perhaps with some silica gel inside. The gel will absorb excess moisture but you'll need to replace it periodically. It's reusable, though - drying can be done in an oven at 250-300°F for an hour.

I've fired old paper rounds in the last couple of years....

Yeah, they were in Dad's ammo box, probably older than I am

As a caveat, treat ALL fixed ammunition, regardless of age or visual condition as if it had just been loaded.
Two anecdotes: A family owned a FLINTLOCK and someone decided to dribble a little powder in the pan and touch it off. They did, killing a family member with a ancient charge and ball that had lain quiet all those decades.
In a second less extreme, but satisfying case: A burglar found the WW2 1911 owned by a retired marine. He tried to shoot the owner, but the vintage hardball didn't fire. He was looking DOWN THE BAREEL when that 50 y/o primer said 'oh, is it time?'

I'm a sucker for a story with a happy ending like that, especially this time of year.

-Blast

Hey Desperado, buying Buck shot... seems to be a bit more expensive than the birdshot..

I'm with Chris, I've certainly degraded in the past few years.

My dad has my grandfather's 16ga. Probably last fired in the late 40's, maybe early 50s. With about 30 rounds. I think we're both a bit afraid to find out if it works!

Yeah, especially if it has color case hardened parts. Sounds like it's time to clean that one and hang it on the wall.