The Adult Forums are just that. They contain material that is not suitable for players that are under 18 or are too immature to handle such. In order to accept these forums you MUST be over the age of 18 and must demonstrate that you are capable of handling the contents within these forums.
If you wish to join the Adult Forums, go to "Usergroups" (right under the ST logo), and press on "View Information" for the Adult Usergroup. Then you can petition to the moderator to join this group.
**Keep in mind that the moderator has the ability to reject your petition even if you are over 18! The decision is based on his judgement.**