I don't know how you feel about old age, but in my case I didn't even see it coming. It hit me from the rear.

The joke reframes aging as something that arrives abruptly show rather than gradually: you don’t “see it coming,” it “hits” you. The humor depends on surprise and on the bodily metaphor—“from the rear”—which adds a mild shock value and suggests indignity, vulnerability, and loss of control. Beneath the punchline is a recognizable truth: people often experience old age less as a planned transition than as a sudden realization prompted by physical change or social cues. Diller’s comic strategy is to deflate fear with exaggeration, making the inevitability of aging feel momentarily manageable by laughing at its unfairness.