20. Is ‘aging begins at gastrulation’ literal?

It is a shorthand for a measurable transition: in available epigenetic clock analyses, biological age decreases early in embryogenesis and then begins to increase later, with the minimum occurring around mid-embryonic development (around gastrulation). Because aging biomarkers and clocks are proxies, the exact developmental timestamp may shift with data and models; what matters is the existence of a minimum and a subsequent rise. This view invites mechanistic questions: what processes drive the rejuvenation phase, why does it end, and can we manipulate its depth or duration.