I was given a kitten when I was three. When I was eighteen, this cat decided to move in with a neighbour who was temting him with fresh fish, chicken, etc. He would occasionally strut back in to ours but would go back to the neighbours when the food was not up to his new level! The neighbours knocked on our door one day after about a year, and handed back my poor Whiskey who had lost weight and basically lost the plot - he was wandering aimlessly in circles. I had to take him to the vet to be pts and it was the worst thing I had ever done - the first animal I had ever had to take.
I could have punched their lights out for letting him get like that and not get vet treatment.