To start - yes, it made a huge difference for him. He was 15 1/2 when I started the medication. Although the studies were done on "middle aged dogs I decided a senior could do it too. I didn't see it myself (I was too close) but my friends and neighbors ALL saw a huge difference in his behavior. A bit more bounce, more active, huge appetite difference. The most important change was when I took him for a check up 4-5 months ago, his blood work showed a complete resolution of his kidney failure (mild to begin with) which meant that the drug was giving him benefits - a lot of vets feel that aging in dogs (and people) has a large autoimmune factor and this drug is for autoimmune conditions.
He also continued to eat well, wag his tail, try to get up on my bed and was generally very happy.
All of that being said, he had a stroke 2 weeks ago at 16 1/2 and I had to put him to sleep. If I had it to do again I would and I would start the medication years earlier. A big dog of age 16 1/2 is a real oldster! But it did help his old age.
Now, after 43 consecutive years with 1-2 dogs at a time, all living into the teens, I'm a bit at loose ends.