We had one like this recently.
Just the one adult male cat involved, but the couple were living in squalor. Poor Raffles was thin, flea ridden, but the worst thing was his teeth. Heaven knows what they'd been feeding him but he had to have all his back teeth removed. He was only around 5-6.
He was found laying outside their house and we think he'd just 'given up'. They had a broken off cat flap through which you could see into their house - it was beyond words.
They opened the upstairs window and said Raffles wasn't theirs but everyone knew he was.
There is nothing you can do in those circumstances except remove the cats. We're not social workers and if people chose to live in those circumstances, no one can force them not to. Remember dear old My Trebus from the Life of Grime...