The oldest KK we've eaten was a "rescue" over 4 years-old & she was fine, though a wee bit on fatty side - good for sausages though
I don't see any problem with giving them a "loving home" before eating them - IME they are usually so glad to get them off of their hands you never hear from the folks again (especially the ones with no provenance!).
I keep hoping that one day folks will start treating KKs as pigs, smaller than other breeds, but pigs none the less - NOT bl**dy fashion accessories
... rant over