He explains that Skull Kid placed a curse on him, turning him into a child and he has been too ashamed to show his face to his fiancé. In addition to this, his Sun Mask, which was meant to be part of his marriage ceremony, was stolen by Sakon. Helping him retrieve his Sun Mask and complete his marriage to Anju forms a basis for one of the largest side-quests in the game.
Kafei seems to be friends with many of the townspeople since he's the mayor's son. The fact that he can show off his Sun Mask, which is basically like a wedding ring, is a slight indicator of that. Anju's mother doesn't seem to like him though. Kafei is also friends with Cremia, who runs Romani Ranch with her sister Romani.
We are never shown Kafei's face as an adult, but in the end, it's indicated that he returned to his adult form.