This is not a law, this is a voluntary regulation undertaken by the motion picture industry.
The first amendment is a restriction on Congress, and does not apply to the private Motion Picture Association that maintains the film rating system.
> why they can have speech abridged
I don't think that's true of federal law. There are cases, such as in school, where more restrictions are permitted to the school. But those restrictions are not based on age.
After 18 you can choose whether to be in school or not. Those restrictions are voluntary for adults but compulsory for minors (who do not get to choose whether they go to public school, private school, homeschool, or just pass the GED)
This has always disgusted me about public school.
No better way to erode rights and democracy than ingraining absolute tyranny in children. I was never search but I was convicted and punished many times on zero evidence, just some authority assuming I did something.
Wrong I’m afraid. Minors don’t have full rights, that’s why their bag/locker/car can be searched randomly, why they can have speech abridged, why they can’t see NC-17 movies(edit: scratch that one). The supreme court has weighed against minors many times.