Wiktionary
demethylation
n. (context chemistry English) The removal of one or more methyl groups from a molecule.
Wikipedia
Demethylation
Demethylation is the chemical process resulting in the removal of a methyl group (CH) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen atoms.
The counterpart of demethylation is methylation.
Usage examples of "demethylation".
Maybe the demethylation is due to cumulative damage to the DNA itself, maybe the methylase enzyme becomes less active in the course of time, maybe something else.