Hi Mt,
Agree with O. This is not allergy related this is probably an Upper Respiratory Infection and needs an antibiotic. LOL! At 18 think your little man can benefit from just visiting the vet for a check up. When we get old S--t happens, I for one sure know that to be true.
Keep us posted and congrats on giving him a wonderful healthy life for 18 years--that is a real milestone.
CML
Wow he has sure been well looked after to be a grand 18 years old....but with age comes a lower immune system and this sounds a lot like kitty has an upper respiratory infection URI. This needs a Vet visit and an antibiotic, allergies with cats appear on their skin, like sores and hair loss, not like with us and the symptoms you describe. Let us know ok