Between 12-15 (?) it's a dog's prerogative to "slow down" a bit. And many do get stiffer joints, guess that comes with the territory.
You sound like you are doing the best for him, with the vitamins and supplements (Glucosamine & Chondroitin, I guess?) He still gets those walks, and that's a good thing -to keep as active as possible while respecting those sensitive joints (he might not want to go so far, or so fast as he did) Keeping active and getting good exercise and fresh air is one of the secrets of keeping as well as possible. And if you're not sure "Who is taking who" on those walks, (!) then he does sound quite active and lively.
Of course it can be difficult to tell with an older dog, if the "slowing down" thing is natural, or whether he has a problem. Dogs, being naturally stoical, can make it hard for us to distinguish sometimes, if there's a problem going on or not.
A really good way to tell if there are any underlying health problems is to get the vet to run a Senior Blood Panel on him. This will give a pretty comprehensive view of his general health, kidney and liver function, etc, and give a chance of getting early treatment should any issues show up.
There is a differance between 12 and 15 however 12 is still senior for a dog and still would be causing many problems due to old age.
Your vet is able to run tests on him if You are willing to pay the expense however he is getting old it is expected for him to slow down.
Unfortunately with old age there is not much a vet or an owner can offer, it is sad but it happens to us all.
If he is having problems with pain associated with arthirits because of his old age I know where I live there is an injection that can be given to help an older dog be in less pain and be able to move around a lot.
It does come down to the price the owner is willing to pay however and their financial situation as it can be quite expensive.
Just make sure he is comfortable, on the right diet, give him love and attention and make sure he has a warm comfortable bed.
Be aware of weather changes obviously as in old age he may experience the cold a lot more.
Best of luck..
My dogs always seemed to tank around age 13. I dread that coming year with our next dog in line for old age problems.
I agree with the Senior Panel suggestion. Most dogs are considered seniors around age 7, and that's when I start asking my vet for full labs on my healthy dogs. I don't want to get caught short with another shocking diagnosis of pancreatitis, liver or renal failure that is too late to help.
Dogs are as individual as people when it comes to old age. Some slow down to the point they sleep all day and finally pass quietly in the night; others keep on truckin' until something finally brings them down. We all owe a death. It's what comes before that day that counts.
OOPS! Forgot to add that yes, massage is absolutely a great thing to learn for your dogs (and horses and most other 4-legged critters). Go very shallow at first and beware of sore spots - usually over the hips and shoulders in older dogs.
Our dogs love a massage. One got the idea right away and got right into it. She passes out in less than 5 minutes. The other kept looking at me like, "Just WHAT do you think you're doing?!" A few minutes at a time and a few days later, he now begs for it by leaning his entire body on me and pushing his head on my hand. LOL!
A 12 year-old dog would be similar to a human who is anywhere from about 70 to about 90 years old. That is a lot of leeway between 70 and 90, but the dog-to-human age comparison depends on the breed of the dog and what is the normal longevity in in that particular breed. In general, the smaller breeds tend to have longer lifespans, although there are exceptions.
Many, many breeds just don't live much longer than about 12, even if you are lucky. Some, like the mastiffs, great Danes, and most of the other giant breeds, rarely even live to 12. They often pass away at more like 8 or 10, and sometimes even earlier. So even if your dog is only 12, he is an older dog, no matter the breed. If he is 15, that is very, very old for just about any breed of dog. You rarely hear of 15 year-old dogs of any breed.
BTW, I'm in my mid-50's (which is maybe 8 or so in dog years?), and believe me, I've slowed down quite a bit compared to my younger days. I agree with others who have posted, your dog is bound to start slowing down at some point, and it is a normal part of the lifespan. If he is still happy and wants to pull you along on walks, he sounds like he is doing well.
I wouldn't push him. I invite you to just enjoy this time with a happy, healthy older dog, and let him take as many naps as he wants to. As he continues to get older, he will probably start sleeping more and more of the time, and his walks will get shorter and shorter, and all of that will be very normal.