I am sorry to hear about the loss of your dog. There is no doubt that they become part of the family. I always give my dogs the best that I can and make sure that they have their shots up to date, micro chipped, well fed and cared for. I had a really good vet years ago that told me "you will know when it is time". He was right. For me, I have large dogs and they don't live anywhere near 18 years though I sure wish that they did. When it is time, I make sure that I'm with the dog when the vet gives them the shot and make sure that they are comfortable and at peace when they go. Letting them suffer may make an owner feel better, but it doesn't make the dog feel better. You did the right thing. It sounds like Scooter was one of the lucky ones to have a good home for so long.