8. Organismal Aging Begins After the Point of Minimum Biological Age

Damage begins to accumulate already during development (14–17). For example, a 5-y-old child is older (has higher biological age) than a 1-y-old because it has more damage (e.g., more somatic mutations, more damaging cross-links, lower capacity for regeneration). However, the 5-y-old functions better in nearly every dimension, indicating that functional capacity is built while aging unfolds in the background. Nevertheless, organismal aging does not start at conception. Its exact start is partially metric-dependent as various damage classes are reset during early development. Operationally, aging starts after reaching the point of the lowest biological age of the organism (14–16). In mammals, this happens when the organism exits its natural rejuvenation phase during early embryogenesis and starts accumulating deleterious changes in a net direction. Biomarkers of aging support a pronounced decrease in biological age during early embryogenesis, followed by an increase later in development and further in adulthood, implying the existence of a minimum biological age (ground zero state) after which aging proceeds.