Sad topic. Advice asked for, so here it is.
In MY opinion, People can understand why they are suffering, and CAN still enjoy life with Pain.
When your dog no longer is enjoying its life, and seems to be wondering why you, its leader, is not taking away the pain, it is time.
I know it sounds vauge, but every time I have had to make the decision, it seemed pretty clear to me. Note clear does not mean easy.
We have a blind OES with many health problems, but he is enjoying his life (still). it was suggested we put him down years ago. He is a lot of work, but scratch him behind the ear, let him sit at your feet and even put your feet on him like a footstool (hey, he likes it, seirously) and he is happy, just being close to us and playing what little bit he can.
BTW, many dogs do not get enough minerals, Wont fix the age thing, but I find giving my dogs, a couple alfalfa pills a day seems to help with a lot of their joint aches and pains. Aspirin seems to help OES's eye problems from hurting so much, and seems to reallyreduce his blood overclotting probs, and in summer, Thyme seems to keep off the flies and bugs some.
good luck and enjoy the time you have left with blue....