1
general::
Have a romantic or sexual relationship with someone much younger than oneself, as in The old editor was notorious for robbing the cradle, always trying to date some young reporter. [Colloquial; first half of 1900s]
American Heritage Idioms
2
general::
Phrase(s): rob the cradle
Fig. to marry or date someone who is much younger than oneself. • I hear that Bill is dating Ann. Isn’t that sort of robbing the cradle? She’s much younger than he is. • Uncle Bill—who is nearly eighty—married a thirty-year-old woman. That is really robbing the cradle.
McGrawhill's American Idioms And Phrasal Verbs