robbing the cradleAMERICAN
Cradle-snatching is the practice of having a sexual relationship with a much younger partner. The woman is even older than his mother. It's cradle snatching! There'll always be those who accuse you of robbing the cradle. Note: You can describe someone who does this in British English as a cradle snatcher or, in American English, as a cradle robber. The ageing actress is a cradle snatcher, says her toyboy's family. Women who make off with men 15 to 30 years younger are viewed as neurotic cradle robbers. Note: These expressions are usually used in a disapproving way.