Wow, this is a pretty striking result for my Strong Epigenetic Theory of Aging: transplant an old heart into a young animal and the heart is rejuvenated. But transplant a young heart into an old animal, and it quickly ages to match the recipient. This shows that biological age is not simply something an organ accumulates locally as irreversible “wear and tear.” Instead, to a large degree, it is a state imposed by the organism-wide environment. The authors saw bidirectional remodeling of both DNA methylation and gene expression, with old hearts in young hosts moving toward youthful programs and young hearts in old hosts moving toward aged ones. That is exactly what you’d expect to see if aging is a regulated, system-level state where the epigenome is acting as the writable control layer that tissues continuously adjust to the organism’s age.