Proper labeling can be helpful, but the music/movie content regulators go a lot further than labeling in their restriction on content. Basically teens are forced to torrent content that seems obscene to christians until they are 18. This is a form of market regulation that seems more likely about pushing christian cultural homogeneity onto a group that can't politically object, than simply labeling what the content is.

One is not explicitly barred from buying a product with too much fat or sugar due to their age however. Cigarettes and alcohol sure, those have proven societal and health implications, but the analogy falls short on the basis of the buyer not being restricted from purchasing that which contains the nutritional info

This is a very good analogy. Media is input for your mind in the same way food is input to your body.