I'm a little concerned about this.

I'm quite happy to accept, regarding Aisha, that the cultural standards were different at the time, and not just in Arabia. That's a non-issue. But...

Firstly, suggesting that "if you're able to fulfill the duties of marriage, and are capable of it - then you can get married" in the context of the modern world is just plain wrong. The obvious reason, of course is that you have no idea whatsoever whether someone is able and capable of fulfilling the duties of marriage (whatever they may be) until they actually try. A subsequent annulment, that may or may not happen, could not undo the damage. In Mohammed's time the luxury of waiting generally wasn't available; it is today.

Secondly, there is no basis whatsoever for denying someone is a 'child' just because they are capable of having sex and having children. None of the countries you list has an age of consent as low as nine, or anything like it .. and in places like Holland it is a recognition that kids of that age will inevitably experiment with sex, not some sort of social and moral approval that they do. It's a balance of setting the reality against what is morally desirable, and protecting 'young adults' from those who would exploit them. I would point out that in Holland the actual age you can get married is 18, except in very exceptional circumstances.

Thirdly, I strongly suggest you expand your research to cover some psychology. That statement is, sadly, complete nonsense.