I don't have first-person experience because my digestion doesn't work very well no matter what I put in it, but I have many friends who have had to restrict their diet as they got older. I have one huge foodie friend, in his sixties now, who over time lost the ability to eat wheat, onions, dairy, yeast, and anything fermented. It's entertaining trying to cook for him. (He's been getting wheat back lately.) I have another friend who developed allergies to non-fish meats and cow's milk.

There are others who have more limited and life-friendly issues as well, I just picked two of the more dramatic ones. I know enough people like this, or with lifelong food allergies, that I'm something of an encyclopedia of how to cook for unusual requirements.