What do you mean by "elderly"? I expect you mean those with dementia, serious mobility problems or depression (or all three). I doubt if their problems started when they were perfectly able but didn't actually enjoy cooking or see the point.
It's laughable to think that people in their sixties should keep cooking just so they don't get malnourished when they're very elderly.
I've been eating the way I do now for about ten years and I'm not malnourished (I also have low fuel bills). I've just been tested for blood glucose, full blood count, ferritin, folate, lipids, liver function, kidney function, pituitary, potassium, sodium, Vitamins D and B12 and had a bone density scan. Every one, except blood glucose, is in the standard range, so "not cooking" (or, to be more precise, minimal cooking) isn't doing me any harm and I have every intention of continuing like this until I'm too decrepit to open the fridge door.