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. A: "Grandpa has had trouble remembering our names lately." B: "Yes, your grandfather is in his dotage these days it seems."

be in your ˈdotage

(often humorous) be old and not always able to think clearly: Sarah moved back in with her father so that she could look after him in his dotage. ♢ Sometimes the kids talk to me as if I’m in my dotage! OPPOSITE: (in) the first flush of youth, enthusiasm, etc.