In many cases, the policies already exist, but the companies are too lazy to enforce them properly. Or in the case of Twitter, really selective.
I think the best solution is to teach internet safety, and actively prevent kids from going on. Every corporation in the world wants you plugged into a screen because it’s easy to advertise to you and get data that way. The net isn’t safe, and we all need to unplug.
As for who, I don’t think we can trust corporations because they’ll want to promote content that helps them and get rid of content that could hurt them. We can’t trust the government because they can control what content is or isn’t acceptable to stifle their political opponents/decent. I think we need to turn back regulation, and teach people to take responsibility for their own safety online.