As a noun senility
is (uncountable) senescence; the bodily and mental deterioration associated with old age.
As a verb decrepit is
Other Comparisons: What's the difference?
(uncountable) Senescence; the bodily and mental deterioration associated with old age.
(uncountable) The losing of memory and reason due to senescence.
(countable, archaic) An elderly, senile person.
- He was entering his years of senility and not liking it a bit.
* decrepid (obsolete)
Weakened or worn out from age or wear