15. Gompertzian Mortality Is Common but Not Universal

Gompertzian mortality (an approximately exponential rise of hazard with age, e.g., doubling roughly every 8 y in humans starting at the age of 28 y) is a population-level regularity, consistent with a rising probability of system failure as the deleteriome accumulates and protective redundancy erodes (29). Gompertz’s law can fail during early life, when extrinsic hazards dominate, when sample sizes are insufficient to resolve age dependence, and when organismal design and renewal keep intrinsic hazard nearly flat for long periods. A flat or declining hazard with time, as observed in species such as naked mole rats, does not mean the organism does not age; rather, it may indicate that the system prevents deleterious changes from manifesting as a rising risk of death. Gompertzian mortality is a useful measure of population aging in systems where it manifests, but it should not be universally conflated with aging, as mortality is not always a robust measure of aging.