He's so old his blood type was discontinued.

This is a one-line hyperbole built on the language of medicine and bureaucracy. By joking that someone’s “blood type was discontinued,” the speaker treats age as if it were a product line phased out by an institution—an absurd mismatch that heightens the insult. The humor depends on the audience’s familiarity with standardized blood typing (A, B, AB, O) and the impossibility of it being “discontinued,” making the claim a deliberately impossible proof of extreme old age. It fits the tradition of American stand-up and gag-writing where age is exaggerated through pseudo-scientific or administrative metaphors to create a quick, punchy put-down.