The 10,000 hour rule is not just about 10,000 hours. If you read Gladwell's book (or better, the research that Gladwell seems to cite), you find that there are

The 10,000 hour rule is not just about 10,000 hours. If you read Gladwell's book (or better, the research that Gladwell seems to cite), you find that there are important things that go along the 10,000 hour rule, probably the most important being a great teacher with a great teaching technique being there for you all the time (such that he/she can give you instantaneous feedback), who keeps pushing (vs. sticking to basic things for too long), deliberate practice, etc. Perhaps the reason why Mozart immediately sticks out is it's hard to conceive in your mind someone becoming that good so quickly. The reality is, with Mozart and probably this prodigy, he has parents who're exemplary in their respective domains (music and art), and they both live in a nurturing environment that served them well. (Oh, and as for Mozart: he didn't become good that quickly. He started composing early, but the compositions that solidified him as a genius came much later on).