Chase will be 10 in September and is still eating his regular food, the same food 3 year old Lark eats, the same food he's been eating for several years. He stays around 85-90 pounds and has since he was about a year old, I just feed him less now than I did before. I supplement with glucosamine/chondroitin with MSM (specifically Dasuquin or Cosequin with MSM per my vet's recommendation but many others are used here as well), Vitamin C and fish oil. He's been getting them for about 3 years, not for symptoms but because I had to put Lark on supplements for some limping, so they both got the same things.
Chase loves going out in the snow, will lie outside in a blizzard given the opportunity, and has fortunately not had any problems related to cold, exercise, or anything else. He does seem a bit stiff in the back end but when he had a tooth extracted recently and had to be on Rimadyl and Tramadol for about 3 days, he didn't seem to run around any more happily than he usually does so I don't think he's in particular pain. I have several orthopedic beds or memory foam beds available for him but right now, as he usually does, he's lying on the rug between the sofa and one of the very expensive beds we got him.
If Lulu is doing well on her current food, I don't think you need to change it- senior foods don't have enough glucosamine or chondroitin to make a therapeutic difference. If you need supplements, I'd do it separately. JenC said she's started feeding ProPlan Bright Minds to one of her older girls. I'll be interested to see if it seems to make any difference cognitively. I only recall one other person on here saying they give their dog any sort of supplement for cognition.