If I am not too mistaken Sweden is alot like Norway, and in Norway we learn english from we are eight years old, therefore all teenagers are competent english speakers (I for one is 15 years old).
A important factor is also that all movies etc are english.
And yes singing in english is better than norwegian at least , I don't know why... songs just sounds better with that language. And to me swedish is a bit weird, as it is more like a parody of Norwegian (swedish, danish and norwegian is very simmilar) and is therefore a bit weird, when I listen to for example Vintersorg's "Ödemarkens son" the entire album is in swedish and although I really like the album I don't like the singing as much since it is in swedish, but it seems that swedish is easier than norwegian.