There's a phrase with guinea-pigs "Well-to-Dead" which sums them up , they can hide their illnesses really well and many times they'll just "go"
Only one of the pigs DD and I had was a piglet (and sadly he died young , about 18 months) the rest were adults when we got them.
Some died within hours of seeing the Vet ( Guinea-Pig Specialist )
I know with the young boar he was going to die and I wondered if we were mistaken saying we'd take him home ? But if it had been compleyely not in his interest I;m sure the Vet would've said "No we need to euthanise him, now"
Age wise , probably 5 yo (ish) difficult to be sure .
But they were pretty good right up to the last leg.
One of my sows sat apart from her cagemates , they were in their hutch , she sat on the floorspace - they were in an enclosed shed . That was the first subtle sign .
We had one die with food in his mouth , like mid chew .
And Evil Edna , who free ranged in the dining room when I was a kid . She walked across the floor and just died in the middle . My Dad found her , lieing on her side , on the carpet !
I do think the fancier breeds aren't as hardy as a good solid tubby Smooth , but I guess a lot depends on their breeding stock
It is tough on your DC when their pet dies and unfortunately with guineas there's often not a ot you can do , but accept you sometimes have to make the difficult decision.
(Then get the a-hole comments like £40 to put it to sleep ! It only cost you £10) 