Unfortunately, I've had to have several of our cats/dogs put to sleep over the years for various reasons. Most of them were pets we had for 15-20 years. There were also the younger ones. One was shot with an arrow through his stomach. The dog was in the fence in our back yard. It didn't kill him but he was in a lot of pain and there was nothing that could be done for him.
Another was about 6 years old but had what I believe is now called 'red mange' but at the time they could only keep it at bay with dips every other week. Even with getting the dips (at about $100 a trip) for almost a year, the dog, a golden retriever, was progressively getting worse. He would itch and whine and scratch so bad that he would give himself sores with the irritation. I just couldn't stand to see hear him whining and constantly scratching. We had 3 boys and each boy had a dog. When one of the dogs had to go to the vet for shots, visits, etc., that son would go with me. The golden belonged to our oldest son. After much discussion with all the boys, the decision was to made to put the dog to sleep. I wasn't too keen on our oldest boy, he was about 12 at the time, going with me to the vet to have it done but he insisted he had to be with his dog so he could help him go to sleep.