SOURCE the Babylonian Talmud.

"Rabbi Joseph said, 'Come and hear. A maiden aged 3 years and a day may be acquired in marriage by coition.' Mishnah: A girl of the age of 3 years and a day may be betrothed, subject to her father's approval, by sexual intercourse. ...Gemara: Our Rabbis taught: 'A girl of the age of 3 years may be betrothed by sexual intercourse.' "

acually that is a mistranslation... It mean three years after the coming of age(which is 10 years old) so its like 13. Though it is legaly wrong in the US its about the age that a woman can bear a child.