Far from an expert, but......
My Lil' Lady will be seven (can't believe it) Lucky is eleven (hard to believe).
Let me say this, my vets have never taken x-rays or put my dogs on a senior regime. Never heard of this before. My vets are kind of old fashion in some ways, but I like that. They figure whatever I am doing I am doing fine with the shelties, so keep doing it. For a dog to live til they are 16 you did a great job!!!
On the whole my dogs have been to the vets through the years once a year for their check. Hope I don't jinx them by posting this. I keep them on the same routine. My Lucky is spunky for his age, I am so proud of him. I quess you can call them seniors, but both of mine run, drink, eat. As long as they do I am happy. Your only old as you feel. They must feel pretty good.