in (one's) dotage
In a state of senility or mental decline due to old age. My grandfather in his dotage has lately had trouble remembering our names.
be in (one's) dotage
To be in a state of senility or mental decline due to old age. Just because we are in our 70s doesn't mean we are in our dotage! You'd best treat your children well, for it is they who will look after you when you're in your dotage.