Fathers or legal guardians giving their 9 year old girls to wealthy men is a common practice between the villagers. So, when she grows up she'll be financially secured and actually has a reliable family of her own when she grows up older and that girl will most likely inherit a fortune from her deceased husband. Otherwise, that little girl who might be an orphan and poor will become a fallen woman and practice prostitution until her death. Underage marriages is a necessary tool for some families in order to fight poverty.
Mental capacity and or consent is not necessary when legal guardian/parents approve of this marriage. Because either way, that girl is destined to get married anyway.
It's a terrible practice that should be stamped out. Girls and women should be able to choose who they marry when they are ready to, rather than having their lives dictated by men. All of this is horribly sexist bullshit.