Senility (6)
I believe the answer is:
dotage
'senility' is the definition.
(I know that senility can be written as dotage)
This is the entire clue.
(Other definitions for dotage that I've seen before include "decrepitude" , "when one's powers decline" , "Mental infirmity in old age" , "Foolish later life" , "Senile years" .)