In Afghanistan . The boy is the first cousin of the girl, whose father agreed to the engagement since his sister never had any girls and wanted one, so he decided to give her his 3 year old as a present. This is apprently very common, even to the point that some children are decreed to marry each other even before they are born. You would think that the society, men and women, would be against this entire tradition, since it sucks to be spoken for before you are four, but it's still very popular. The men who end up marrying those women who are selected for them since they are children can always marry a second and third wife if they are not happy, so they have really very little to complain of. It's the women who get stuck, many of which opting to commit suicide to end what must be a very miserable existence.
Oh, and the UN estimates that 43% of all Afghani marriages were like this one. Warms your heart up, doesn't it?