Is it allowed to marry a 9 year old girl in Islam?

I have read many news, poor parents selling their young girls (6 yr to 12 yrs) to old shaikhs in Dubai, S Arabia. These pervert sheikhs young girls and leave them to lead a life of prostitute? Why these Sheikhs, arab men so perverted? How a muslim clergy approve of such marriage? Even Mohammad married 9 yr old Aisha,...isn't it?