Someone who doesn't look their age. Some usually don't even grow facial hair at all in their lives. It was inspired by the forest children in the Legend of Zelda who don't age at all. Many of them find it annoying that people say that they don't look their age but it pays off in the end when others are old and wrinkly and the forest children still look the same as they were 20 years ago.
Guy 1: Man, I had no idea you were 19. You look like you're 15.
Guy 2: It's okay. I'm a forest child. I live forever.