i heard theres a hadith in abi dawud where Aisha herself mentions that a marriage wasn't consummated by arabs until only after a females menses - her menses signifying her adulthood.
How is it possible that someone can have children and be a child? Their logic is confusing and they themselves throughout the different countries in the US, UK and Europe can't agree on a common age for adulthood. Try doing a little bit of research on this, but they differ - ranging from 13 till 18 - So does this mean that having sex in the Netherlands at 15 is peadophilia in the UK? Will anyone who follows the law of one nation be counted as a peadophile in another? Who's truly right?
The answers simple in Islam - if you're able to fulfill the duties of marriage, and are capable of it - then you can get married. However, if it will be harmful [and no-one can bring one shred of evidence that Aa'isha got harmed by her marriage, rather she benefited herself and billions of others in Islam (being in the top 5 narrators of ahadith), aswell as advancing in medicine studies] - which shows that she was perfectly normal and said nothing but good about the Prophet. Whereas a person abused would not praise their abuser willingly.
If someone said that a 9yr old isn't capable of having children or sex;
I would quote the following fatwa from Shaykh 'Abdul-'Azîz ibn Ahmad Ad-Durayhim:
As for the possible negative consequences of a man of such a mature age marrying such a young girl, it is patently obvious. The discrepancies in their capabilities, both physically and mentally, could bring about serious differences between the two of them that could lead to the failure of the marriage. This is something that has been seen and is well understood.
Therefore, I would not recommend such a marriage nor would I encourage it.
Moreover, with respect to what we have said about the legal validity of such a marriage, that refers to the validity of the contract itself. As for the effects of the marriage - such as privacy, intimacy and sexual relations - that is another matter entirely. Such things are permitted only if the girl is able to handle such a relationship without any harm whatsoever coming to. Otherwise, it is prohibited. This is because the Prophet (peace be upon him) said: "There shall be no harm nor the causing of harm."
It can also be seen in the very conduct of the Prophet (peace be upon him). He did not consummate his marriage with 'A’ishah for a number of years on account of her young age.
And from the fatwâ committee supervised by Shaykh 'Abdul-Wahhâb At-Turayrî we note:
The lawfulness of consummating a marriage at such an age is contingent on the maturity of the girl and that no harm would come to her.
So if any harm would come from it, then it is unlawful i.e. harâm.
Q: So why didn't Islam prohibit such practices that are harmful?
Actually, it did. It would be impossible for Islam to have an explicit prohibition on every dangerous behavior from jumping off a scyscraper to smoking, so Islam has provided a single broad injunction to cover all instances of harm:
The Prophet Muhammad pbuh said,
لا ضرر ولا ضر
"There is to be no harming, nor reciprocating of harm." (Musnad Ahmad, authenticated by Al-Albânî)
And Allah knows best.