If a minor who betrothed or married, his actions are null, for the Rabbis did not decree marriage for a minor. And it is forbidden to marry him a woman while he is yet a minor. Rama: Nevertheless, if he already married her, he does not need to divorce her, and she is permitted to remain as is until he reaches the age of majority (Mordechai, beginning of chapter haBa Al Yevimto, and it is implied likewise in Asheri chapter haIsh Mekadesh). Mechaber: Even if he sent her marital gifts (sivlonot) after he attained majority, it is still null and void, for we are not concerned that he sent the gifts as an act of kiddushin (betrothal). However, if there are witnesses that he secluded with her after attaining majority, she needs a get (divorce), for we presume that he had intercourse for the sake of actualizing kiddushin. If he maintained her [as his wife] after attaining majority (and after seclusion, as per Chelkat Mechokek), her ketuba contract is valid and he does not need to write her a new ketuba. This applies only to the basic 100 or 200 [zuz] (for widow or virgin respectively) but she does not have rights to the additional monies written into the ketuba. Rama:If he did not write her a ketuba when he was a minor, when he attains majority he writes only 100 zuz into the ketuba, for at that time she is not a virgin (Hagahot Alfasi, beginning of chapter Mi sheHaya Nasui).
One who engages an orphaned minor, or a minor released from her father's domain, when she can perform miyun and all her laws will be explained in Siman 154. Rem"a: And see later on Siman 169 who is believed regarding a male and female minor if they [later] grew up. If it was doubtful if they were minors or not, it is a doubtful Kiddushin . Beit Din is able to stop from marrying the minor, just like her mother and her brothers have this right, and they are able to put conditions [on the marriage] that if she does miyun , she will lose all that is hers. But they don't have the right to marry off the minor ( Nimukei Yosef Perek Cherech ). And see later beginning of Siman 155.