Greg (32ish) is our oldie these days. His left rear leg, where he has some cystic abdominal tumors, is about useless. He kind of uses it as a crutch when he walks, with his toes knuckled...a habit he learned and kept after opening a surgery site a few times.
His right rear is in decent shape, he has a bit of a hobble/limp with it but he still uses it rather normally. He spends most of his days asleep in a hammock, but does still got up a ramp on the lowest setting. We have a sock stretched over it so he has grip and does not catch his toes in the bars. His H.E.D. seems to be mostly neurological (tremors) rather than arthritis, and he responded well to adding fish oil to his diet and not noticably at adding glucosamine.
It is interesting, in a sad sort of way, to watch how they adapt to their body getting older and more frail. Dom (R.I.P.) used his tail as a crutch/brace when his rear legs got weak, to the point where he opened sores from tissue strain.