The problem with the 10,000-hour rule is that many of its most ardent proponents are political ideologues who see the existence of genius as an affront to their vision of human equality, and will do anything to explain it away. They have a lot of explaining to do, starting with the case of Mozart. As Mr. Robinson points out, Nannerl, Mozart's older sister, was a gifted pianist who received the same intensive training as her better-known brother, yet she failed to develop as a composer. What stopped her? The simplest explanation is also the most persuasive one: He had something to say and she didn't. Or, to put it even more bluntly, he was a genius and she wasn't.
I didn't even know Mozart had a sister! This is especially fascinating because Mozart and Nannerl shared both environment and genetics. What was different about them is the thing that is not reducible to a measurement: their individuality. If we were doing statistics we would call that "error", which of course it isn't in this context.
A thing I worry about with the current fashion of the 10,000 hour rule is ambitious helicopter parents trying to force their children to be this or thus. It can be done. Many famous child prodigies, such as Mozart, are the result of just such a program of accelerated instruction. What is less clear is the terrible price to be paid by the victims of such forced marches.
Charles Sanders Peirce is one of the greatest minds who ever lived in the United States, but the damage done to him by his illustrious father, Benjamin Peirce, meant that he died a pauper, unremarked and unlamented. Benjamin Peirce was chair of the Harvard mathematics department, and he placed his second son through a grueling regimen in which he read all the famous philosophers and then had to deconstruct their arguments for his father, and he composed a history of chemistry by the time he was 11. C. S. Peirce was incredibly gifted, but he seemingly lacked common sense, leading him to vandalize the chemistry lecture hall at Harvard. His precocity and foolishness earned him the enmity of the future President of Harvard, who ensured that Peirce would never work in a university.
Peirce was forced to earn a living doing work for the USGS and by writing book reviews. Just one of the things he created was the Peirce quincuncial projection. A mouthful to be sure, but a beautiful projection of the globe to look like a square:
This is just one many things he created in exile. Peirce's work is still being evaluated, but his reputation keeps increasing as time goes on, But he had a horrible life! He learned everything except how to be a good man, which is really the primary purpose of education. Peirce had to learn this for himself at an advanced age, but how much easier would it have been to have learned it when he should have.