Old is always fifteen years from now.

Cosby’s line plays on the relativity of “old.” Rather than being a fixed age, “old” is framed as a moving target—something people project into the future to avoid identifying with it in the present. The joke captures a common psychological habit: we measure ourselves against an imagined later self, preserving a sense of youth by redefining “old” as always just beyond reach. It also hints at how cultural ideas of aging shift with perspective; what seemed old at 20 feels young at 35. The humor works because it exposes a self-deception that is nearly universal.