The ESRB has no legal bearing, meaning you can feel free to let your teens play M-rated games if you want to. They might run into trouble trying to buy a copy from a local retailer, as most will not sell to anyone under 17 if the game is rated M (or under 18 for AO).
But even this is not against the law. If they buy it without being ID’d, the only way they can get in trouble is with their parents, not the law.