If I start any kind of company, I cannot just invent new rules for society via ToS; rather the society makes the laws. If we just make a simple law that states minors are not allowed to access the web and/or access any user generated content (including chat), it won't need to be enforced by every site/app owner, it would be up to the parents.
The same way schools cannot decide certain things for your children (even though they regularly over reach...).
We need better parenting. How about some mandatory parenting classes/licenses for new parents? Silly right? Well its just as silly as trying to police the entire internet. Ban the kids from internet and the problem will be 95% solved.