a senior moment
An instance in which one forgets something or acts strangely or foolishly, humorously attributed to one's advanced age. I thought Mom knew Joe's phone number, but she's having a senior moment and can't remember it either.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a senior momentINFORMAL
A senior moment is an occasion when someone, usually an elderly person, makes a silly mistake or forgets something. He is 69 in February and sometimes has a senior moment, when the flow of words dries up. Note: This expression is used humorously.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012