We know lots of things that contribute to ageing, as others have said, “ageing” is a broad term. There is evidence that we accumulate more cells that do not behave as they should and are functioning improperly. Some researchers say these cells have mutations in their DNA, essentially that the instructions for making proteins (the machinery that makes cells function) are broken. Some say that the cells don’t function due to epigenetic issues (the bookmarks on the DNA that allow it to be read or not). These junk cells then cause lots of issues, they may divide too much or interfere with nutrient or oxygen transport, and they may just do nothing. There is debate about what comes first and which factors are the most important.
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