Most 3rd graders are already on the same level as Puljujärvi. Meaning they are good enough to say: "My name is Jesse. I like hockey. I'm very happy Edmonton." It's almost as if he's skipped every lesson after 3rd grade. And there's just no way a person living in Finland is not exposed to English because of music, movies, tv programs, games and internet are often if not mostly in English.
When you compare him to someone like Patrik Laine, an average speaker who's learned to speak in school, the difference is quite huge.