I had my dog on a variety of supplements, from glucosamine to Zeel, a homeopathic arthritis inflammation medication. What I heard about glucosamine is that it won't build any more cartilage, but supports what's left from deteriorating further. So, theoretically, you should have them start before they start showing symptoms.
Had I known what I know now, I would have tried him on other supplements like serrapeptase and hyaluronic acid.
Not much helped Chevy in the end, he was on Metacam for the last couple of years of his life, and while I don't know if it helped, I didn't stop it. It's so hard to see them get slower.
you might want to try iherb.com. if you type in "dogs", plenty of joint health supplements for dogs come up.