Lots of insight already. I will add my kids' examples as another data point. Both of my kids love music, but for DS, the timing was tricky. He started piano at 4.5 yo, after some group lessons and a lot of urging from the group lesson teacher. I was going to wait till he was older, but the teacher just felt that he was ready and he was so talented, etc. So I started him with private lessons; he rebelled within a month -- he was not ready at all to do the boring daily practice. So I let him quit right away. I thought it was absolutely not necessary for him to take formal lessons at 4.5, especially if it would make him lose interest in music. DS started piano lessons again at 6, is still taking lessons now at 16. He is a wonderful pianist.
DD11 started piano at 5.5 yo. But since she was 3-ish, she was picking out tunes on the piano by herself, she learned how to read music and could sit down with her brother's beginner books and play all the songs in them for a long time. She didn't need an audience, she simply enjoyed the music and the act of playing piano. She begged for piano lessons since she was 4.5 yo, and it took a year to plan things out with the teacher before she finally started lessons. However, after she started lessons, formal practice was built up very gradually. For the first year of lessons she probably practice 15-20 minutes each day. It increased to 45 minutes in the second year. Now she practices piano about an hour a day. But because she also plays violin (also started at 5.5 yo after a lot of begging from her), and she participates in a lot of musical theatre/opera performance, the practice time is appropriate. Her piano teacher also feels that it is adequate even if she wants to focus on piano later on.
So I guess my points are that every kid is different, music learning comes in many forms besides sitting down and practicing the teacher's assignments, and that kids can well have different learning needs at each stage...
