Some Indian Casinos in California are 18+ as well. Morongo, Barona, and Santa Ynez come to mind. I've played at both Morongo and Barona and both are run rather efficiently and the facilities are top-notch IMO. You just cannot check into a room at either if you are 18-20, you must be 21. Other than that everything else is fair-game (alcohol aside).

Morongo is where the recent Poker SuperStars Invitational Tournament was held.