Peace to all,
I’ll give you all a short example of 2 nine year old girls, one from Somalia and another from Kenya. Before I begin, both of these girls were forced into marriage, and this is forbidden in Islam as Allah said:
"O ye who believe! Ye are forbidden to inherit women against their will….” (4:19)
The nine year old from Somalia was a developed girl, not in mind but physically. She looked more like your average 15 year old. However, the Kenyan girl, was a tiny little girl, and had the body of a five year old. Both girls got pregnant, and the Kenyan girl died during the birth of her child while the Somali one survived. The point is that, even though both girls were forced into marriage, girls grow at different paces and because of that reality, it is up to the parents not to marry of a girl, no matter the age, who is not developed enough.
The second point is that since the marriage can only take place when the girl approves, it is highly unlikely that a nine year old, or even a 13 year old would want to get married, unless their parents put on a guilt trip.
Islam, does not rule out something which is permitted if the girl and her parents allow it. When a marriage takes place, the parents should always look out for the welfare of their child and this means her health. It is the parents’ responsibilities and God expects this from them. If there is a nine year old that can marry, like Aisha, and that everyone is fine with the arrangement, then it’s only fair that such a marriage should take place.
Now days, nine year old girls do not wish to marry, and on many occasions they are forced into marriages, which leads to death because of difficult pregnancies. This is not the fault of Islam, but the fault of the parents. The permission should be sought from the daughter, and then the parents should make the wise decisions. God will ask them of their responsibilities on the Day of Judgment. So if they fear Allah, they will do justice by their daughter.
In concluding, I do not disagree with young marriages if the right precautions take place and permission is sought from the daughter.
Peace and Blessings be to the Believers