"Senior moment" was the first one that occurred to me.
>Senior Moment is an idiom used to describe the phenomenon of a momentary lapse in memory common among senior citizens.
Two early citations:
Please ignore this person. He is obviously suffering from a senior moment (or a brain fart, you choose).
—Joan Ellis, "Probably the most stupid question to ask in this group," rec.food.cooking, May 3, 1996
Evelyn Weinstein, who works with 35 nursing homes in Nassau County, came to the conference for herself and her patients. "I really want to learn what I can for our people," she said. "And I've had a few 'senior moments' myself."
—Lilo Stainton, "Impressive turnout for workshop on memory," Daily News (New York), November 15, 1996
In 1998, "members and friends of the American Dialect Society at their annual meeting" voted "senior moment" as the most useful new term.