Major retail chains and consoles require an ESRB rating of M or lower. No rating, no sale.

If you are selling the game yourself, and its on the PC, then don't worry about it.

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Oh, just because I've seen so many rants on the ESRB, let me just make some things clear: The ESRB doesn't ban or censor games (it only gives ratings). ESRB ratings are only guidelines, it isn't illegal to sell M or even AO rated games to minors. The ESRB is a private company (like the Movie rating board).

Some people in the US government want to eliminate the ESRB so they can create their own rating system (you do NOT want this to happen).