You're uh... seriously misrepresenting some aspects of the situation. The ring was not "known" for it's underage workers- 10 out of 90 girls in the ring were between the ages of 15-18 and 15 is the age of consent in France.

Also, it wasn't really a "brothel" so much as it was a loose collective of girls and women who worked for a certain man. De Niro got caught up in it because he was introduced to a porn star by the dude who ran the escort service. Her name was Charmaine Sinclair- she was overage and he didn't pay for her services. Two other women mentioned him in passing but they never mentioned having sex with him and he claimed he shook their hand.

Even, if he had, had sex with the prostitutes there, he didn't commit a crime, as the other person pointed out. Prostitution is legal in France. He was questioned because the method of procuring that the heads of the ring used was illegal, not because he was going to go to jail in any way for what he may or may not have done.

Comparing him to Cosby who drugged and raped numerous women is really, really stupid IMO. I'm not saying his actions were necessarily morally right but I feel like you're being pretty disengenous, honestly.

Here's an article that goes into it more: https://www.thefilmhashery.com/articles/the-nightmare-that-robert-de-niro-endured-during-the-shooting-of-ronin