What wee beauties! The look like little bears! My girl just turned 10 and is still bouncy and her usual self except for getting a limp often and yelping sometimes, i think she's getting arthritis in her shoulder or something, its horrible seeing the light of our lives getting older :(
Is your dog not eating anything at all? If she eats some stuff maybe her system has just slowed down with age or eating causes her pain somewhere.
Some people ask how do you know when it's time to get a dog out down, apparantly you'll know when they're just lying there, the light's gone from their eyes and their whole demeanor just says 'Pleeease make this stop!', if your pup is still enjoying some aspects of life, pester your vet (or go to another vet) to see if there's any way he could make her happier, without getting her knocked out!
Good luck, kisses to the pups