According to the wiki article on the subject, 96% of child brides are 16 or 17 whilst only 4% of adult spouses are over 29. Whilst it is an issue, the majority of child marriages are between people of similar ages and don’t always reflect abuse.

Also, some child brides are used as anchors for others overseas. Families from places like India and Pakistan, where up to a fourth of their marriages involved an underage spouse at the time of wedding, used immigration loopholes to obtain visas at the expense of the child.

https://www.tahirih.org/wp-content/uploads/2015/03/REPORT-Tahirih-Survey-on-Forced-Marriage-in-Immigrant-Communities-in-the-United-States.pdf