Why do grinder and cutting discs expire anyway?

It’s a valid question. They’re hardly going to rot, or go stale. But there is a very good reason why manufacturing regulations like EN12413:2007+A1 mandate that cutting discs be marked with a use-by date.

The discs are bound together with resin, and that resin absorbs moisture from the air. Even if they are kept in a sealed container, the air in there with them will be enough to begin breaking down that resin. They are built to resist breaking down for 3 years. After that, the chance of disc failure becomes too high for safety.

Would it be so bad if a cutting disc did fail? I could just put in another.
Disc failure is incredibly dangerous. Cutting discs spin at thousands of RPMs (depending on the disc and the grinder/cutter). When one ‘fails’ at that speed, it flies apart with the force of a bullet.
In all seriousness, this is how workers lose fingers and eyes, even if they are wearing the right PPE.

How do I know when my cutting discs expire?

That depends on the disc and the manufacturer. Larger cutting discs with a centre ring usually have the expiry date printed on that ring in a month/year format – anything that says this month and year is already expired! Smaller discs do not typically have centre rings, so you’ll have to look on the disc’s label, on the disc itself or on the packaging it came in. In any case, this date will be at most 3 years form manufacture. If you know those disks are pushing 2½ years from purchase but can’t find the expiry date, you should probably discard them. Better safe than sorry.

What does this European safety standard say anyway?

European StandardEN 12413:2007+A1, Safety Requirements for Bonded Abrasive Products applies to bonded abrasive products like most cutting discs, but not to coated abrasive or superabrasive products. Those are covered elsewhere. It sets standards for the way these materials can be designed and manufactured, and requires safety testing. It also requires all resin-based bonded abrasive cutting discs be marked with a use-by date that is no more than 3 years after the date of manufacture.