We usually talk about age as a single number. You blow out one more candle each year and you select a single age bracket on surveys and medical forms. Pediatric growth charts, vaccine schedules, cancer‑screening guidelines, and risk scores for heart disease all start by asking the same thing: “How old are you?” In many ways, the entire culture of health revolves around the idea that one number can stand in for how far along you are.
Chronological age is largely interpreted as a kind of master setting for a single process unfolding across the body. At the same time,…
