That is crazy!
The older girl is very mature emotionally and socially, but she lacks some technical intelligence and awareness
But, does it matter if she has all the base skills to learn well? Or does it matter if one has all the technical skills at that age, but can't learn much else well? I think having core skills means being able to learn more later, so the potential is higher. Having secondary skills early, might not be able to make up for the lack of core.
I got two little hacks to learn all my cousins, nephews/nieces. First, at age 8-9 - fast reading techniques, if they practice few months, their reading speed will at least double. The time you save in your whole life is huge, once you get this. Then, as they reach 14, fast-learning techniques and memory-enhancing skills. Also, time-savings.
Also, the secret sauce, learning more similar stuff will get you quicker where you want. Let's take foreign language, the Latin group has French, Spanish, Romanian, Italian, Portuguese. Each of them, if you learn them all. will take less when you do it, as they got similar words. The same works with musical instruments, the more you learn, the better you get at feeling the music. But I think piano or guitar must be first. In my humble opinion.