[ENGNR]

My limited understanding is that the repair processes also play a big part. For example bones become brittle because stem cells no longer fill the gaps as quickly as they appear. The body also fully replaces every cell every seven years. If they could find a way to just restore the repair mechanisms a whole host of problems could slowly start to disappear.

[reasonattlm]

Bones also suffer from other processes that damage the structural properties of the extracellular matrix such as cross-linking by hard to break down sugar compounds. There is also a process of growing cellular dysregulation wherein the ongoing bone remodeling favors destruction over construction, and that is distinct from stem cell decline. Every tissue is impacted by multiple forms of damage beyond loss of maintenance via stem cell function, some of which, like the cross-linking, accumulate slowly and cannot be repaired by our biochemistry. Clearance of some sort will be needed.

The body does not replace all of its cells; different tissues have different turnover rates. Many of the cells in your central nervous system will be with you for your entire life. There is research to suggest that even some of the individual proteins in those cells are never replaced either.