As someone who sees people on a regular basis with a variety of ages who are starting from basically scratch learning an instrument, I always do a running assessment of their skill set development. People come with varying abilities, or lack there of. Typically they are strong in 1/3rd and weak in 2/3rds. Left to their own devices they will nearly always consciously and sub consciously avoid addressing their weaknesses in favor of the more pleasurable results that working in the areas they are good at can get them for the effort they put in. My approach is to look at the big picture and coach them with this in mind.
The issue of 10k hrs practice, from my POV, is that the best gains to be made are when you involve as many components as possible, such as the ones I have noted above (My list is not absolute nor exhaustive). Paying attention 'globally' to your development in an ongoing manner makes for better results than just zooming in on 1-2 things. The thing I find particularly with children is that they are so clearly defined by my list. While some adults have closed the gap somewhat on some of their weakness simply due to a 'maturing' process, they ALWAYS carry their basic quotas and tendencies with them into lessons.
My approach to teaching/coaching means that I am continually spiraling around these areas within the context of a chosen instrument. Experience tells me that not everyone has an equal capacity to either understand concepts or execute physical skills. For what ever reason, either through lack of nurture or through real natural inability, some people will NEVER be as good as others because they never had the potential in the first place. Sure there are 'late bloomers' who intermittently come out of the wood work displaying extra ordinary skills or perception beyond what appears to be their experience, but they are by far in the minority. This is why part of any good teachers involvement is to help a person accept that they will only ever be as good as their 'make up' will allow them to be. Enjoying the process of personal development through the journey of learning to play an instrument is an absolutley wonderful thing. Encouraging a misguided belief that you can be anything you want if you just work hard at it is nuts.
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