Mozart’s sister as quoted in Wikipedia…..
He often spent much time at the clavier, picking out thirds, which he was always striking, and his pleasure showed that it sounded good. […] In the fourth year of his age his father, for a game as it were, began to teach him a few minuets and pieces at the clavier. […] He could play it faultlessly and with the greatest delicacy, and keeping exactly in time. […] At the age of five he was already composing little pieces, which he played to his father who wrote them down.
The kid in the clip below does not quite top that. Still, it is a little unnerving to watch him play (they make him do to much talking at the start of the show to demonstrate how cute he is or something. But if you endure it, you will get to see him play.)