At that age most of us are still trying to master the proper use of the fork. His first symphony came at age eight:
When the rest of us were learning how to ride a bike. When Mozart and his father embarked on an extended tour of Europe for three years, when Mozart was between seven and ten years old, one of the things that they did was give public exhibitions of Mozart's uncanny skills. He would show up, sit down at the piano and offer to sight read any piece of music. In London he was reported to have sight sung an Italian aria while simultaneously reducing the orchestral score and playing it on piano. At eight years of age. Without seeing it with your own eyes it is hard to imagine the impact of this. But we can try. Watch this clip from Amadeus:
First of all, the characters are all rendered a bit stereotypical for the sake of creating an entertaining farce. The Emperor was likely a rather more formidable person than is depicted here: he was after all the absolute ruler of the Austro-Hungarian empire. Also, all the musicians, including Salieri, were highly skilled professionals, not the bumblers depicted. Another misstep is Mozart's reaction to anything having to do with Italy. He spent a lot of his formative years there and spoke Italian very well. I'm sure that, when greeting Italian maestros of music, he would have responded in Italian and he certainly loved Italian opera! He wrote a lot of it! But where this clip really falls down is in the levels of skill it displays. In reality, Salieri's march would have been a far more accomplished one. But the supposed demonstration of musical memory and compositional skills Mozart shows here are far too rudimentary. Imagine him doing something a hundred times more skilled. And now, imagine him doing it at nine or ten years old!
All that Mozart's character does in this scene is correct a clumsy 6/4 harmony (I think he replaces it with a vi chord?) and then just adds some rudimentary melodic decoration which he repeats in octaves. Then he adds some Alberti bass and turns it into a different piece. I'm sure that Mozart age eight could have done better!! Here is the Andante K. 15, written when Mozart was eight and a much better piece than Salieri's march:
So, now, replay that whole scene from the movie in your mind, but instead of Mozart being a fully-grown man of perhaps thirty years, imagine he is eight years old.
That is what is so remarkable about Mozart. He seems to have been a fully mature musician from his earliest years.
When he was ten years old he wrote a quodlibet in seventeen movements, the last of which is a fugue for orchestra:
As his father wrote to him once, "Wolfgang, you can imitate anything."