In many states its legal if approved by a judge
http://marriage.about.com/cs/teenmarriage/a/teenus.htm
This is one I see that specifies a 13 year old
New Hampshire: Individuals under the age of 18 may not marry in New Hampshire. Brides must be at least 13 years of age, and if under 17 years of age, will need parental approval and a waiver. Grooms must be at least 14 years of age, and if under 17 years old, will also need parental approval and a waiver.