Dogs age faster than we do unfortunately, and as dog lovers we may outlive several of "man's best friends." I know I certainly I have had several wonderful canine companions in my life that I've outlived so far. As your dog ages there are several things you can do to make their lives more pleasant, just as you change your life somewhat as you age, hopefully making it more pleasant. For example, at almost 50, I don't stay out late partying and chasing the opposite sex as often as when I was 20 (and when I do my wife gets mad!). I recover more slowly and gain weight more easily too. My habits have changed and your dogs do too.
You owe it too your dog to make their life as pleasant, easy, as well as healthy as they age. Here are a few ways to do exactly that.
You may want to change your dog's diet to a food for older dogs. They have different nutritional requirements and usually are less active just like older humans, needing fewer calories. Otherwise they will tend to put on extra weight which is unhealthy, just like for humans. There are plenty of dog foods designed for older and less active dogs.
They also need activity as they age, but may become more lethargic just as humans do. I take Tyler for a walk a couple times a day to get him some needed exercise. When he was young we let him run around, but now unless we make a point of it, he'd rather just sleep or lounge. He is very happy to engage with activity though as long as we come along too.
You dog may also need supplements and pills when they age, just as humans do. I take two pills a day, unlike when I was younger, and so does my dog Tyler. His vet has suggested something for his arthritis and we add two tablets to his breakfast every day. I strongly suggest going with their vets recommendations here.
Older dogs are also less nimble and less sure footed. Tyler has trouble sometimes getting into the car. Eventually we may get him a ramp although he is doing OK here right now with some coaxing and sometimes just a little help.
Stairs are also hard for older dogs for several reasons including arthritis and other degenerative diseases. You can make stairs far less difficult by making them non slip with rugs.