I agree with Momofmany here. (I may be overstepping my knowledge here - I'm still learning).
Questions: Is your cat keeping down water or not? Is it just food that you see or not? If he/she isn't keeping down water you need to get him immediately because he/she can be dehydrated.
It can be a sign of something serious, so please take your kitty to the vet ASAP for tests. Its not too uncommon for older cats to have problems.
I had the same problem for a short time with my 17 year old Sphinx. It was found out by trial and error (all blood tests and xrays came back fine) that he had only food allergies.
Hope this helps.