Ouch, that hurt!
Stephen is one of the few honest and good people left on the earth, so due to his honesty, he informed the waitress that his bill is less than the actual amount. Which he had thought was some kinda mistake but got corrected as that wasn’t the case.
Here it was indicated how fast he was aging, as he is just 39 years old and they assume him as a senior citizen. That’s going to hurt and this kinda stuff will be in your head for a long duration.