Dotage is a noun for the time in one’s life when one is aged and not as strong as before. This weakness is physical and sometimes mental as one loses memories and the ability to recall information. This noun is usually used with a possessive pronoun, such as her dotage, my dotage, or their dotage. The plural is dotages.
Senility is the state of being when one is senile, an adjective for the specific loss of one’s mind as one ages. Senility is a mass noun and therefore does not have a plural, even though senilities seems to make sense.
These words are synonyms, and while one might be more prone to connote physical decline versus mental decline, the nuance will be lost on most readers.
Examples
I’m not exactly in my dotage, but I do carry the weight of a lot of advice about how to behave in the garden — some of it conveyed gently in beautifully written columns, some of it shouted unsolicited from the sidewalk. [The Denver Post]
Kellock, Brown, Hamilton and Evans are older than 30 but not so far into their dotages to be dismissed as candidates for the World Cup next year. [The Telegraph]
One of us is suffering from premature senility, between the FA Cup and me, though without confirmation from the clinic I wouldn’t dare tempt fate by suggesting which. [The Independent]
After the service, I tell Gallion about my mother’s passing, her plummet into senility and the unremitting fear that I will suffer the same fate. [The Washington Post]
The adjective “senile” is now considered disparaging toward older persons, with the result that many editors no longer permit its use. For the noun referring to age-related loss of mental faculties, “dementia” is now preferred. Although Alzheimer’s disease is the most common cause of dementia, these two terms aren’t synonymous. “Senile dementia” is redundant and politically incorrect, but still frequently encountered in older literature.