When my youngest was a few months old, I took him along to the symphony when I was chaperoning one of the older children's field trip. Several people, including myself, noticed that he moved to the off beat in the music and responded to the different cadences. When he was walking, he would crawl up on the stool of our piano and pluck the keys and create simple songs. As he got older, they became more nuanced. We bought him a keyboard for his third birthday, and he's been composing music ever since. He chose percussion in band in middle school, so he has an electronic drum set as well. He doesn't like to practice, and I had no goal for making him practice or learn conventionally, so it's completely self-exploration.
