The younger waiter is disgusted by the old man. An old man is a nasty thing. The same thing may have been said by the old man when he was young. One might even conjecture that the old man chooses to be deaf rather than to face the nastiness of caducity and hear the words of disdain spoken by his juniors. Another tool used by Hemingway in this story is the image of Nothing.
Nothing is what the old man wants to escape.
The Nothing is a relentless monotony, unbroken by joy or sorrow. It is unending emptiness without comfort or companionship of man or God. It is the senselessness of each heart-beat that is just like the last and refuses to give in to death. The old man's loneliness is empty. His days of retirement without useful work or purpose are empty.