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.
I have run two Olympic 'A' standard times over the past 12 months and with the time I ran at the African Championships last week I know my speed and fitness are constantly improving so that I will peak in time for the Olympics.