PHeMoX: So far, in all the US states, it isn't illegal to sell M-games to minors (or, technically AO games). The same is true for movies, music, books, art, etc.

Some states (California included) are trying really hard to change this for games but, so far, they haven't managed it (free speech and all that).

Note: While it isn't illegal to sell AO games to minors, it is illegal to sell them "pornography". So, selling "Manhunt" is legal but selling them the Playboy Sim game isn't.