[treya]

10,000 hours may be a vague rule of thumb, but it says nothing about what is going on, and it's not just a matter of practicing. Learning, knowledge, wisdom, etc. all come from neural growth over time - your brain responds to thinking/concentrating/practicing by growing in the areas of the brain that were used, and this growth accumulates over time. Even in much older people the brain is plastic and is producing new neurons in addition to growing existing synapses, albeit at a gradually slowing pace. A corollary to this is, don't expect to understand things immediately - keep working at new concepts and skills. If you persevere, over time you will understand them and become proficient.

It's important to note that "practice" can't be something like, for example, simply taking tennis lessons - you must be challenged and struggle for it to have an impact. I would also include desire as being essential - it must be something you truly want to do, not something you just go through the motions for. Also consider that your physical health, regular exercise, diet and quality of sleep are intimately tied to cognitive growth.