Epigenetic alterations
The instructions get misread
Every cell carries the identical book, and a set of bookmarks tells a liver cell to read the liver chapters and a skin cell to read the skin ones. With the years the bookmarks slide. Cells begin reading pages that were never meant for them, and doing the wrong job in the wrong place.
This is where the frontier is loudest, because in animals the bookmarks have been pushed back where they belong. In people it is not a treatment yet, and anybody selling it today is selling ahead of the evidence. What is on the table now is dull and real: sleep, movement and not smoking all track with slower drift when it is measured.
The science, in one sentence
The chemical marks that decide which genes are switched on drift with age, so cells lose the identity of the tissue they belong to.