Our boy is 12 and is really slowing down 
. The vet prescribed him ibuprofen type medication as she felt he was probably suffering with arthritis type issues in his hind legs. We are just finishing a two week trial of it and tbh there has been no discernable difference in his mobility or sprightliness.
I am normally the one who walks him so my view of his gait is normally me looking down on his back iyswim. However DH had his lead yesterday and I was watching at a distance and noticed his back seemed to be sloping down slightly like a German Shepherd, as if his back legs were struggling to support him. He is a very big solid 'blocky' style of Retriever and he always weighed in at about 44kg, but he's actually lost about 3kg in weight as well.
If you have had older Goldens then how did their old age pan out, and did they follow a similar sort of 'going downhill' path? I plan to make another appointment this week to discuss other medication options. He still has a good appetite and his temperament is as lovely as ever.