How common is child marriage in the United States?

Approximately 6.2 of every 1,000 children surveyed had ever been married. Prevalence varied from more than 10 per 1,000 in West Virginia, Hawaii and North Dakota to less than four per 1,000 in Maine, Rhode Island and Wyoming. It was higher among girls than among boys (6.8 vs. 5.7 per 1,000), and was lower among white non‐Hispanic children (5.0 per 1,000) than among almost every other racial or ethnic group studied; it was especially high among children of American Indian or Chinese descent (10.3 and 14.2, respectively). Immigrant children were more likely than U.S.‐born children to have been married; prevalence among children from Mexico, Central America and the Middle East was 2–4 times that of children born in the United States. Only 20% of married children were living with their spouses; the majority of the rest were living with their parents.

That is from Alissa Koski and Jody Heymann, via the excellent Kevin Lewis.