cradle-snatcher someone who has a romantic or sexual relationship with a much younger partner (thefreedictionary.com)

What is it called when a younger girl likes an older man?

A person with such a sexual preference is a gerontophile. The word gerontophilia was coined in 1901 by psychiatrist Richard von Krafft-Ebing. It derives from Greek: geron, meaning “old person” and philie, meaning “love”.