In Kansas, actually, common law marriages are legal when the girl is just 12...even now. Here's some more information:
"In addition to statutory law, Kansas is one of the few states in the union that recognizes Common Law Marriages. The age required for Common Law Marriage in Kansas is lower than that required for a statutory marriage by K.S.A.23-106, above. For Common Law Marriage, the female must only be 12 years old, the male 14."
Oh my goodness... In New Hampshire it is illegal to walk with your shoes untied. The point being - common law "marriage" would require a 12 and 14 year old to live together outside of marriage and that just does not happen now a days so it is not a legal issue. In New Hampshire they don't prosecute or arrest the man with his shoe untied even though it is on the law books.