I had only 1 experience with a older dog. He was a dachshund and my first dog. I had to put him down at a tender age of 15 years old.
He had mellow down( a little). Had to put him down because of cancer.
My oldest yorkie is 7 years old and still a little spitfire and I hope she never mellows out. My father has a yorkie (9 years old) and she hasn't slowed down either.
I think why people think a dogs will begin to get meaner is because when they are older they get physical pain from being old. When my aunt Maltese mix was 17 year old she suffered from extreme pain from arthritis and would snap if you touch her the wrong way. But can you really blame her? But is not always the case. When my dachshund was suffering from cancer and I could tell he was in exteme pain the last few months of his life but he never show teeth and always wagging his tail and was always eager to be petted. It practially killed me to have to put him down but my vet held my hand and told me it was the best for him and cried with me. Okay, I'm getting sad. I'm going to stop writting now.