Certain boards on the forum must be explicitly opted into. You can do this by asking in the Opt-In Board Requests thread.

Currently, the only opt-in board is Mature. To access to the mature channel you must be physically, legally 18 or over. If you break this rule we will ban you from the forum. If you are a system containing both adults and minors, you are personally responsible for making sure that younger members stay out of the mature channel. If you break this rule in this manner we will permanently remove your access to the board.