Response: Maturity does not occur in everyone at the same age and time. So asking whether someone of a certain age is maturity cannot be answered. Many societies, particularly those in the past and in Islamic traditions, begin to give adult responsibilities at 5 years old. So due to such an early age, they mature faster. In today's society, it's taught that a 5 year old is immature automatically, so they mature much slower. So it is very possible for a 6 or 9 year old to be mature. I was. So was everyone in my entire family. We were raised that way. Every one of my siblings had to be placed on a higher academic program than normal as a result. My elder sister started school at 2 or 3 years old, instead of the standard 5 years old. We all cooked family meals, performed chores, and many responsibilities that an adult is accustomed to doing at an early age. All studies show as well that the earlier you school and discipline a child, the more likely they become mentally mature. So while it is definitely unlikely or hard to believe to you that a 6 or 9 year old can be mature, those who have been raised differently and are not a product of the stereotypes of modern society know otherwise and are a product of such.
As for your question, there is absolutely nothing morally wrong in marrying a 10 year old girl, if such a girl is mature and does not disapprove of the marriage.