Mandated reporting laws vary among the US states, but this is a common situation: If a girl of 14 goes to a therapist and reports that some cousin (during a rare visit) sexually abused her, the therapist is required by law to report this to the authorities, who will conduct an investigation. The girl and her family will be interrogated at length. They are innocent parties, but they do not have the option of being left alone.
In contrast, if woman of 30 goes to a therapist and speaks of a rape that happened when she was 28, she is in control of the situation. The therapist would be committing a grave breach of professional ethics to tell police about this without the woman's permission.
There are arguments to be made for and against an adult victim deciding to make a report. If she reports it, she is likely to be subjected to intense questioning on a painful subject to determine if she is telling the truth. If there isn't a strong legal case, the rapist will remain at large and she will have gone through an unpleasant process for nothing. If she doesn't report it, she gets to maintain her privacy but perhaps feels some guilt that she is doing nothing to protect some other woman from her same fate. But a vital point is that the choice is hers. Her rape was against her will, but she at least retains the power to decide whom to reveal it to.
If the girl is underage, the rules are different. Society has a strong interest in stopping child sex abuse, but in its zeal it often inflicts another kind of abuse on the child. She is stripped of any choice. She cannot keep this matter private any more. She may have great difficulty obtaining an attorney who works for her interests alone. The younger a child, the greater the role parents should play. But parents do not have the option of huddling with their daughter and affirming her decision to keep the matter private either. Law enforcement decides whether to interrogate and investigate.
Sexual activity with anyone underage is a crime, but some young teens participate enthusiastically, some consent with ambivalence, and some are of course forced into sex. Any spot along this continuum is possible. A grown woman who knows she consented but regrets what happened can discuss this non-crime with a therapist at length. A minor has no such option. It is both a crime and a mandated reporting crime.
Many minors who understand these consequences and want to maintain their privacy have absolutely no one they can turn to -- no teacher or coach or therapist. Maybe they could get away with telling a priest if they are involved with a church, or if they are sophisticated enough, maybe they could tell a lawyer, though this is not a profession known for helping people resolve their feelings. If their parents are understanding, perhaps they could tell their parents. But parents may not be understanding, or they may be part of the family dynamic that allowed the abuse to happen in the first place.
Adult women would be justifiably outraged if they were denied the right to discuss rape with therapists in confidence. Why is it OK to deny this same right to minors?