In one's dotage (6)
I believe the answer is:
senile
'in one's dotage' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for senile that I've seen before include "Suffering from ailments of old age" , "In one's dotage" , "Suffering from weaknesses of old age" , "Affected by old age" , "Suffering from mental weakness due to old age" .)