Many children have recently been married in the US despite a minimum marriage age of 18. This can be evaded, however, if there is parental consent to the marriage. In many cases the child can be coerced into marriage, but there is no procedure for investigating this in the US. There can also be exceptions when there is judicial consent.
Read more at: http://www.nytimes.com/2015/10/14/opinion/americas-child-marriage-problem.html?_r=0