The equivalence of biological aging and clock time-keeping
It would probably be prudent to mention: All processes—chemical, biological, measuring apparatus functioning, human perception involving the eye and brain, the communication of force—everything, is constrained by the speed of light. There is clock functioning at every level, dependent on light speed and the inherent delay at even the atomic level. Thus, we speak of the "twin paradox", involving biological aging. It is in no way different from clock time-keeping. Biological aging is equated to clock time-keeping by John A. Wheeler in Spacetime Physics