Actually it was the norm in the early 20th century for girls of 13-15 to marry men in their 20's onward. However, that doesn't mean that should be our standard today. Women also weren't expected to do anything but raise a family and take care of the home and help with farm work and couldn't vote.
Most betrothals of little girls are to men who much older than they should be and in Yemen, a culture geographically close to Israel and taking on some of the same traditions, these child brides are given to their spouses around 8 or 9 and even though these men in their 30's are suppose to wait until they have had their first period to have intercourse, 9 times out of ten they just rape them and everyone turns their heads.
Women for centuries have been barely one step above property. In such a culture, marriage and bearing children was a woman's salvation. So, without any other future to pursue, she was married as soon as they saw fit to achieve her duty to bear children.
Today, women in most of our cultures are not property and we have the ability to become educated and pursue other avenues besides becoming wives that are supported by husbands and bearing children as our saving grace. There is nothing wrong with being a wife and bearing children, it is a wonderful thing, but we also allowed to develop more fully intellectually, emotionally, and spiritually which is a very good thing because that creates a marriage of equals who can work together to more fully display what the image of Christ and his relationship to the church is. Understand, I am saying equal in respect to being equally acknowledged as human beings and in reference to Christ being acknowledged as co-heirs with him.
If we had not been raised up by him to become spotless before God and righteous just as he is then we could never as the church become his bride. So, if we are to display this sort of relationship in a marriage then we start with the premise that men and women are worthy of equal respect and are equally matched to work together and display the witness of Christ and His church.
Women have been raised up because God has raised them up when they were nothing more than a man's property. It is a big deal that women are no longer forced to marry in their early teens and that is why it is so regrettable that this young woman and her mother are not allowing her to fully become the woman God wishes her to be.
Without being established in Christ first and having a firm spiritual foundation, a marriage is already off to a rocky start. If this girl is has been having sex since the age of 12 then she barely knows who she is because she has had her identity intertwined with this grown man. If he knew Christ at all and genuinely loved her then he would be willing to wait for her to grow up and treat her properly as well as establish himself in Chrit so he could be a spiritual leader in the household.
Obviously, neither one of these children are self-sufficent and think their sign of adulthood and maturity is having sex. They don't have a clue what it is like to hold down a full-time job, pay bills, and deal with the relationship issues that can rock a marriage to the core. They haven't had opportunity to take care of themselves much less each other and more concerned with their "love" than making themselves into respectable independent mature adults.
No one living with their parents should get married. That one thing shows a lack of responsibility and maturity that no married person should own.