Pornography
In the law of most countries, those people who have not reached the age of majority, or minors, may not buy, let alone sell, pornography of any sort.  More importantly, there are laws against showing minors naked in photographs or films meant to arouse others sexually or showing them engaged in sexual acts.  Such material is called "child pornography," and is considered not just illegal, but a serious crime.  The laws against child pornography (also called "child porn") are meant to protect children against people who "use" or "exploit" them. The laws also require those who make pornographic photos or films to keep records to prove that their models, actors, and actresses are over the age of legal majority.