We've not had a dog grow very old but we had a cat who needed to be put down because of kidney failure. It was horribly difficult and even though it was 15 years ago, I'm getting weepy a bit remembering. For our kitty, he began to urinate everywhere very suddenly and then there was blood everywhere. It was fast and sudden within 2 days and I had just brought my first baby home from the hospital.
Now our shepherd mix doggie is 11 and has had some recent health problems. He had surgery this week to get a tumor removed and his eye duct flushed (two separate issues) and they are concerned what they removed from his paw is cancer. We won't know until the biopsy comes back. My kids don't know that after seeing it they are very concerned. And all 3 are very attached to the dog. This is the dog we got when my dc were 4.75, 3, and in utero. I am a little afraid for our family when he goes but I'm praying that they are wrong about the tumor and that we can move on to letting him grow old peacefully.
I have cried a few times this week at the THOUGHT that our beloved pet isn't long for the world so I'm having a hard time putting myself fully in your shoes.
I will be praying for you.