I think food is a big factor too - I realize I have a basset hound, so joint issues are predominant anyway, but my vet told us to get her on glucosomine/chondroitin about 3 years ago (when she was 3). I never would have thought to do so without that tip - she was so young!
Even without the vet's opinion, go look around the PetSmartCO - the food companies have gone crazy lately and have breed specific food, age-specific food, energy-level-specific food, etc. Try something new out.
This also might just be a seasonal slump for your pooch, sensitive stomach, or allergies, or something else minor that can be managed very easily. Small changes in diet or supplements may very well increase pup's energy level, and improve quality of life for you both.
There are no studies proving the worth of glucosamine/ chondroitin sulfate. In college, my older D participated in such research (on horses but the results were negative nevertheless). I'm resisting putting the almost 8 year old collie on a senior diet until he needs it, as shown by the presence of crystals in his urine.
I thinking keeping our pets at optimal weights is very important for their health. This can't be done by caloric restriction alone usually.