Not a very good argument. The games already have ratings and purchase restrictions. We also have ratings and restricdtions for movies.
The ratings and restrictions on video games and movies are both completely voluntary, industry-adopted standards, not enforced by the government. There is no law on the books that says they must rate them, or that it is illegal to sell material with any rating to someone of any age. The only laws that might apply are on the order of “contributing to the delinquency of a minor”, which aren’t related to the content, but the effect it might have.