The hard part about a dog slowing down is that it does not happen on a Tuesday. It happens across six months, eight months, a year, in the form of a dozen small things that you only notice in retrospect when you are looking at old photos and realizing the dog used to bound up the stairs and now they take them one at a time and you cannot remember when that changed.
By the time most owners ask the question of whether their dog is starting to age, the dog has usually been aging for a while, and the question is not whether but what to do about it.
The signs that actually mean something
Some slowing down is normal, some of it is not, and the difference matters because one of them is the dog growing into their senior years gracefully and the other is something specific you can usually do something about.
Stairs are the biggest tell. A dog that used to fly up and down the stairs and now hesitates at the bottom is almost never just getting older, they are almost always dealing with hip or back pain that is bad enough to make them think twice about a movement they used to not think about at all. Same thing with jumping onto the couch or the bed, especially if the dog tries, fails, and walks away rather than trying again.
Eating slower or skipping meals is another one, and most owners assume it is the food, when it is usually the mouth. A dog with a sore tooth, an inflamed gum, or low-grade dental infection is going to eat less, eat slower, or eat from one side, and they are not going to tell you any of that because dogs do not complain about their teeth.
Sleeping more is the trickiest sign because some of it is just normal aging and some of it is something specific. The version that worries me is the dog that used to greet you at the door and now lifts their head from the bed and watches you come in. That is not just sleeping more, that is energy not being there to spend, and that is usually a nutrient story or a thyroid story or a gut absorption story.
What actually helps, in order
The first thing that helps, and this is going to sound boring, is weight. Senior dogs that are even slightly overweight age noticeably faster than ones at the right weight, because every extra pound is more pressure on joints that are already wearing down, and most owners are bad at honestly evaluating their dog's weight because the dog has looked the same to them for years. Get a vet's read or look up a body condition score chart and be honest with yourself.
The second thing that helps is the daily basics, the multivitamin, the food topper, the gut support, the omega oils. Senior dogs need higher density of nutrients and lower volume of food because they are not burning as many calories, and most regular kibble is built for the opposite math.
The third thing is the mouth. Get the breath checked, not because of the smell, but because the smell is a symptom of bacterial activity that is doing damage you cannot see. If a full anesthesia cleaning is not on the table because of the dog's age, there are still things you can do daily that move the needle.
The fourth thing is movement, but the right kind. A senior dog needs more frequent shorter walks, not the same long walk you used to do, because joint pain compounds across distance but not across time. Two 15 minute walks beat one 45 minute walk for an older dog every time.
What does not help, no matter what anyone tells you
Switching to a fancier kibble does not fix what fancy kibble cannot fix. Adding a tenth supplement on top of the nine that already are not working does not solve the problem, it just makes you feel like you are trying. And waiting to see if the slowing down is just a phase is the move that costs the most time on the back end, because the things that help work slowly and the sooner you start the more time you actually have.
Most owners want one big thing they can do that fixes it. There is no big thing. There are a few small things done every day for the rest of the dog's life, and that is the actual answer, even though it does not feel like one.