He's 20. But he has been treated as a child his whole life because that whole island is mostly elderly people. It's hard to mature when there are so many people around you treating you like a child.
That's actually what you call a childlike innocence, my dear. He has been on an isolated island where there are nothing that could engraved evil things in his mind, thus maintaining his naive nature and trust anyone he meets.
Then why did the girl - more or less the only other young person there - act completely normal for her age?
Okay, the uke seems deadass dumb. He talks and acts like a child. How fucking old is he?