That man is so old, he's older than his birthday.

The line is a comic exaggeration built on a playful logical impossibility: everyone is, by definition, older than their birthday (a birthday marks a date, not an age). By stating it as if it were a remarkable fact, the joke turns a truism into an insult, implying the person looks or seems extraordinarily aged. In performance, it functions as a quick “old-age” put-down—part of a broader vaudeville and nightclub tradition of hyperbolic one-liners—where the humor comes from the mismatch between the statement’s faux-precision and its obvious meaning: “he looks ancient.”