I don't consider 7 to be old in a dog that should expect to live around 12 years, average. I say this because when my first Lab reached 7 the pet food store started pressuring me to buy "senior" food and for the same reason I think you are quoting, that too much protein is not healthy for an "older" dog's kidneys.
Both my Vet and the breeder said "baloney" to that.
My breeder never feeds anything but an adult maintenance diet from weaning to death. If a special diet is required, determined by annual Vet visits and attentive ownership, then she might feed a diet tailored to declining kidney function.
My last girl showed some signs of arthritis at age 8. We started her on Cartrophen Vet injections and they were like a miracle.
But then I read an article about 50 year old humans not being able to be "weekend warriors" ie. exercising hard on weekends and working office jobs midweek. I reasoned dogs might be the same so I INCREASED my girl's midweek exercise so that she was not doing all her longer/harder outings only on two days, one after the other. For us this meant about an hour skiing, snowshoeing, hiking, bicycling a day with at least 2 hours and sometimes 4 on Wednesday, and the weekend. IT WORKED. Took her right off the Cartrophen.
Now, I did have her on Glucosamine/ Chondroitin/ MSM 1300, twice a day, whatever human brand was on sale as per Vet. I kept that up and if I forgot a day it was apparent, I think. This regime held her till her back went at age 12.5 when she then had to go on something much stronger for pain management. But till then she was a happy girl and did all day hikes with us on vacation and portaging on canoe routes. On one of those longer days I did often give her an aspirin as well. 325 mg of aspiring once in a while is OK, says my Vet.
Good luck with your girlie pup.