[khedoros]

> Myself I practiced the piano very deliberately in my late twenties and it didn't lead to improvement.

You had a teacher who provided a lesson plan and guidance, until such time as you could follow the plan on your own and self-correct mistakes? As another element, were you constantly pushed to do things that were just out of your reach? His claim is that 10,000 hours of practice/learning structured in that way would make you an expert piano player. Somehow, I suspect that if you hit 90% of his requirements, he'd "no true Scotsman" you and say that it wasn't "real" deliberate practice.

Personally, I'm dubious about Ericsson's claims too. Some people have more limitations than others do, either physical or mental. Plasticity has its limits.