I lost 2 earlier this year, 15 and 16,
Now we are dealing with a 15 yr old and every night we ask each other "Is he in pain?" No
"Does he enjoy part of his day" Yes.
The first girl, we waited a bit too long. I knew this, but forgot - the vet told us her brain was shutting down. She had seizures (for the last 8 mos) and was losing the ability to control her rear legs. He said if all else is healthy, they start at the rear of the body, and gradually lose control of the body, working up to the brain.
So, we are letting the boy enjoy what he can. He sleeps a lot, but he did always like his pillow. When I show him where his food is, he enjoys eating it. Today he was just circling in a small circle. He has a bit of 'dementia' so my husband just grabbed him to sit with him for a bit.
Like Zande said, dignity with no suffering. You just evaluate on a dog by dog basis. The second girl was 16 and the vet said "You just know."
Give yourself permission to decide, and know you've loved him all these years, You just know.
We let her cross the rainbow bridge.