Children are already accustomed to a world that moves faster and is more exciting than anything a teacher in front of a classroom can do.
I had one companion. He was a teacher from the Ukraine who spoke English so we could communicate a bit. I learnt a few Russian words, but it was hard to concentrate.
I think there are so many ways to become interested in music. I believe signs of sustained interest gives a sense of the right time. Music, if thought of as a language, would perhaps indicate that as early as possible is not so bad. I do believe that a really nurturing first teacher that makes the child love something is crucial.