In the American context, this mostly operates to take away legal rights from teen mothers.
A better alternative is to allow some under age eighteen marriages, but to limit the circumstances when they are allowed:
1. Marriage should only be permitted for someone under the age of eighteen when the bride is either already a mother, or is pregnant and has made a voluntary choice not to have an abortion, and is marrying the father.
2. Marriage under the age of eighteen should require an inquiry approved by a court, that the marriage and the relationship are truly voluntary. And, it should be denied in the face of domestic violence involving the couple.
3. Women who marry under the age of eighteen should have access to free, long term birth control and in the event that they become pregnant, to abortion without anyone else's permission required.
4. We, as a society, owe women in this situation economic support so that they don't make their decision under economic duress.
No situation when a young woman who is not yet eighteen is a mother is optimal. But, opponents of all marriages under age eighteen are making the best the enemy of the good.
Certainly, at some point, our society would be right to incarcerate for a long time someone who has sex with a girl under a certain age.
But, the age of consent is not eighteen and shouldn't be eighteen. Most sixteen year olds have had sex, legal, non-rape sex. Sometimes they are going to have children. And, throwing the man in jail for a year or two for statutory rape does not make the young woman or her child better off in the cases where she is pregnant and choose not to have an abortion or give the child up for adoption.
But, in those circumstances, marriage provides a teen mother with more legal protections than not being married does. It provides her with a right to maintenance (a.k.a. alimony), to a share of her husband's property acquired during the marriage, and various other legal benefits of marriage (Social Security benefits after ten years of marriage or less if she becomes a widow, access to veteran's benefits from her husband, the right to bring a wrongful death action, a presumptive inheritance of all of his property if she becomes a widow, etc.). In the U.S., marriage is not a defense to forcible rape, it is not a defense to domestic violence, and no fault divorce is available in every state, so either party can unilaterally end the marriage at any time. The downsides present in many countries are not present in the U.S.
Yes, someone who marries under the age of eighteen has a 48% chance of divorce within ten years. But, that means that 52% of the time, the marriage does last ten years or more. We certainly don't prohibit everyone who wants to go to college but has less than a 50% likelihood of graduating from doing so.
The divorce rate for people who marry when under the age of eighteen is about twice the rate of those who marry over the age of twenty-five. But, most of that difference is because most people who marry over the age of twenty-five are college graduates who tend to have fewer divorces, while most young women who marry under the age of eighteen are high school graduates at most, and were typically never college bound in the first place. Most women who marry under the age of eighteen, had they not married then, would have married long before the age of twenty-five, probably wouldn't have earned a college degree and probably wouldn't have married someone who did, and would have had perhaps a 40-45% chance of marriage within ten years.
There is certainly no data to suggest that someone who doesn't marry the father of their child but lives with the father of their child outside of marriage is likely to have a more stable relationship than they would if they had married, or that their prospects are improved by waiting under she is eighteen years old to marry. And, the shorter the marriage, the weaker her legal rights are if they divorce.
And, suppose that a sixteen year mother old marries a twenty year old father of her child and they stay married for only eight years.
Is it really true that she is no better off than if she had never married at all?
Her child got eight years of being raised by both parents in an in tact family. She gained alimony and property rights in that eight years and day to day help raising their child. She will be better prepared to be a single parent at twenty-four than she was at sixteen. She will have an easier time managing to hold down a job with a child in elementary school than with a baby who needs constant day care at great expense for her to hold down a job. It isn't the ideal outcome, but is better than the likely outcome if they had never married at all.