But they do. The article argues that molecule ensembles are damaged by thermal motion but there are other ways in which molecules are damaged over time due to chemical and physical stress. Large molecules in the cells are bombarded by caustic chemicals (oxidative stress) and ionising radiation, both of which break chemical bonds. Even without these, many molecules in the cell are only temporally stable and decay over time (e.g. single-stranded RNAs are unstable even ignoring active degradation).
It is altogether accurate (and common!) to describe these processes as ageing on the cellular level.
As you’ve correctly noted, cells constantly repair and replace damaged molecules.
But they do. The article argues that molecule ensembles are damaged by thermal motion but there are other ways in which molecules are damaged over time due to chemical and physical stress. Large molecules in the cells are bombarded by caustic chemicals (oxidative stress) and ionising radiation, both of which break chemical bonds. Even without these, many molecules in the cell are only temporally stable and decay over time (e.g. single-stranded RNAs are unstable even ignoring active degradation).
It is altogether accurate (and common!) to describe these processes as ageing on the cellular level.
As you’ve correctly noted, cells constantly repair and replace damaged molecules.