I’ve just replaced the pads on my CX cantis with some bought from the LBS. They had been in stock for quite a while (years). After one ride I noticed an awful lot of metal shavings around the pad and a corresponding amount of wear to the rim.

Can old pads do this, or I have I just been unlucky in picking up some abrasive dirt?

Maybe your rim has just worn out due to the constant pounding 😯

Some compounds are worse for picking up grit, but it could be that you're just unlucky. What happens is that bits of grit get collected between the rim and pad, and, under braking, chip tiny pieces of aluminium off the rim, making it worse.
Check and clean them out after a few rides, and if it still seems to be happening, try a different pad.

Going to clean it up now and have a better look. Ordered some new pads too.

I was worried that the pads may have "gone off" with age and are now harder than they should be.

Just found a chunk of something hard stuck in the pad. Bumholes.

Rims wear very quickly on cx bikes, 4 – 6 months tops.

oldgit, I.ve been running the same Ksyrium elites on mine for two years+ and theres still plenty of life in them.

I've just bought some Aksiums and they seem pretty beefy in the rim department, the wear indicator seems very deep.
Mine do wear more whilst out training rather than through the seaons racing.

I bought some XTR (parallel-push) V's not long after they came out, and the pad material at the time was infamous for a) wearing quickly b) wearing yer rims quickly. After the first long ride I did, it was pretty obvious by the amount of shiny rim material in the mud around the brakes. Replaced almost immediately by some kool-stops

Yeah, I dunno what Shimano are thinking but their offroad pads are all really abrasive- I recently bought a set and after about five minutes of metallic scraping, decided to put the old koolstops back on

Guilty of putting any old junk on, and come to think of it I did use KoolStops once and they were good.
Kool Stop eagles?
I actually need new pads and was wondering if anyone had first hand experience of those Swiss rat jobbies.

Planet X rims?
They seem quite prone to the whole "metal shaving" thing, not matter how clean the brake pads…riding round rivi in the wet on them was nigh on terrifying!