old enough to be (one's) mother

old enough to be (one's) mother

Of a man, much older than one, especially by 20 or more years (i.e., around the age of one's actual mother). Used especially when referring to a woman who is or might be in a romantic or sexual relationship with a younger person (especially to indicate that the speaker thinks such a pairing would be inappropriate). How could you find her attractive—she's old enough to be your mother! Everyone raised their eyebrows when they met Mark's new wife, as she's old enough to be his mother. But they do seem very happy together.
See also: enough, mother, old
Farlex Dictionary of Idioms.

old enough to be someone's mother

 and old enough to be someone's father
as old as someone's parents. (Usually a way of saying that a person is too old.) You can't go out with Bill. He's old enough to be your father! He married a woman who is old enough to be his mother.
See also: enough, mother, old
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • old enough to be mother
  • old enough to be someone's father
  • old enough to be (one's) father
  • not (all) that good/bad/well/poor/etc.
  • not all that good, well, etc.
  • one's sunset years
  • sunset years
  • twilight
  • twilight years
  • jockstrap