It's not government mandated. It's a defacto requirement as all commercial broadcasters require it but that their commercial choice not government.

What's actually illegal in law to broadcast is very different from what you practically cant due to the theoretically voluntary codes. Even that guidance is broad but hard to argue with "Advertisements must contain nothing that could cause physical, mental, moral or social harm to persons under the age of 18." No reasonable person would argue you should be allowed to do that.