My 18 year old gets a varied raw food diet (normally liver, beef shank, pork bones and some mixed stew meats), with an occasional treat of canned tuna or salmon. But up until she was 12, she just ate grocery store kibble, and then she was on low quality wet food after we were told she was pre diabetic and shouldn't eat any type of kibble and couldn't seem to hold down most raw foods.
The beef and pork seem to really agree with her these days, she is healthier now then 6 years ago, but it did take trial and error to find what her tummy was ok with (she can't eat raw poultry of any kind, although she loves cooked turkey). I think it is more of a concern to keep older cats eating enough for their size and avoiding diabetes etc then perfect nutrition. Sheeba throwing up her (healthier) food was more of a concern to me then her only eating low quality wet food!