I am not ready to call my 12 y/o a senior (
) but she has been getting “Senior panel” bloodwork done every check up at the vet’s recommendation for the last two years, so I guess in the vets’ eyes she’s a senior.
She gets 4 oz Weruva Paw Lickin Chicken split AM + PM. She also gets a little less than 1/4c of soaked Purina Sensitive Stomach in the AM before I leave for work. I don’t know what it is about this food but it is the only food of the hundreds of foods we’ve tried (including Hydrolized, prescription, medicated, etc) that keeps her “idiopathic dermatitis” (per vet) at bay. At our last allergen testing they said she might be allergic to human dander. 
It may sound like a lot of food, but the Weruva isn’t a good source of calories - I just feed it to her because it’s guaranteed only chicken, and it’s a way to get wet food into her. Fair warning about the Purina Sensitive Stomach: it makes cats FAT. I was feeding the whole crew (6 cats) this food for a bit while I figured out a way to keep their feed separate from Miss Sensitives’, and they all got round as ticks.
The rest of my cats aren’t diet sensitive, so they get dry food in the AM topped with a spoonful of Weruva canned, and then whatever canned food was on sale that I bought for PM. Usually the brands are Wellness or Vet-Diet, since those tend to go on sale the most often. I don’t keep to a specific brand of food for my cats that don’t have allergy/diet sensitivities. My main priorities with them are just that it’s a high quality, high protein dry food, and that they get wet food at night when I get home.
If your girl is losing weight or having trouble keeping weight on, the Purina Sensitive Stomach might be a good jump off from your current Purina feed, while you talk to your vet about whether or not you think running any tests is appropriate. My vet[s] think that cats should at minimum have a senior panel done every year, but this isn’t in the cards for all people. Talk to your vet and see what they think.