keratin expression in old mammals

[Iuval clejan]

In old mammals, does the statistical distribution of keratin production
per skin cell shift towards lower production (i.e. most cells produce
less protein), or do some cells just completely stop producting keratin
(because the gene gets damaged  and not repaired) and the rest of the
cells express keratin at a normal rate? Question can be generalized to
other proteins, and to increase in production as well as decrease.
-Iuval