cradle robber

noun1

1. •
   (disapproving) A person who marries or becomes romantically involved with someone who is much younger, or who employs or otherwise engages a young person for a purpose inappropriate for his or her age.

   • "And no mother," he shouted, "can call ME a ‘fortune-hunter’ and a ‘cradle-robber’ and think I'll make good by marrying her daughter!" — 1914