Why does the United States allow underage marriage?

It’s really not a positive phenomenon. I generally disagree with underage marriage because of all the problems it causes. Underage marriage means younger than 18. It causes reduced economic opportunity and educational attainment. Also most teenagers are not emotionally and mentally mature enough to get married. The only exception I could think of is if the teenagers are already have had to grow up fast like making choices about an unplanned pregnancy and they were both close in age. If they wanted to get married to provide the child they made with stability and it wasn’t their parents pressuring them to get married but of their own volition. So say maybe they are 16 or 17. But in general underage marriage is not a good thing. Older teenagers are not children but they aren’t full fledged adults either. 18 is a legal adult to be sure well for most things. Yet adolescence is what it is called until your 20s.