How do you even define content that "might" seriously impair the physical, mental, or moral development of minors. This is so subjective I don't see how you could get away with marking anything that wouldn't in the US be rated PG.

At that point in order to ensure that such content isn't shown to minors you have to require a credit card or an ID because any reasonably kid would figure out how to set his or her age to a reasonable age of majority to avoid being pigeonholed into kid stuff.

Can you give me an example of an alternative strategy that would be legal and more reasonable?