There is an eerie perfection to almost everything Mozart composed, even when he was a child. It is said that he just wrote these things down quickly and without erasures or crossouts though I have seen some. In particular I am amazed by the Orphanage Mass (K. 139) which he composed when he was 12 years old. I suppose that maybe the person we attribute all these works to just copied them from some adult's notebook, but this seems rather unlikely. Otherwise, as for life itself, I am lost for a rational explanation of how this music came to be.
ML/NJ