We're lucky that over the decades most of our dogs have lived to their teens. One or two have left us due to health deterioration like cancer, but others have suffered with dementia. Mostly ours have got to the stage of losing their bearings, or staring at something vacantly, plus loss of bowel function too. My galga Carmela suffered a stroke from which she recovered 90% but my little podenca Bebe is now 18 years old and showing familiar symptoms. So far it's not too advanced that she needs more than medication for thyroid problems, but I expect over the year her brain will let her down more.
I feel very sad seeing old dogs which have been dumped by hunters or owners who aren't prepared to repay their dog's loyalty with care in their twilight years.
Here's an article on dementia in dogs.