Thinning patches of fur

My 15year-old spayed Burmese female has developed 2 patches on her fur, in the hollow above her hind legs but towards the top ... ie near to the spine, first on one side and now on the other. The fur seems about 1/2 the length of her other fur. She seems unaware of it and is neither licking or scratching the patches. I have scratched and looked at the area and have not found dry skin flakes or evidence of fleas etc. Could the patches be due to her advanced age and the fact that she is very thin? She eats well and I feed her Royal Canine Mature Neutered dried food + some packaged cat food in jelly etc.?