I host a game community that has a wide age range. We utilize the PHPBB forum software for our community message board. With the default install, the message board is configured to make it more difficult for kids younger than 13 to register. Because of that we have made 13 our minimum age for the community and will remove users if we find they are younger than 13.

The wide age range is normally not a problem. Younger members interact with older members on a daily basis with mostly good feelings. There are occasional "when I was your age..." or "come on, grandpa, it's the 21st century..." comments, but nothing that is either obnoxious or offensive (it is usually said in a joking context).

That said, yesterday one of our administrators received an email from a angry parent. The email address used was not associated with any of my users and the parent did not provide the username they are complaining saw things they shouldn't. The parent had seen their child on our message board in one of the more adult themed sub-forums. The link they provided was to a thread that I would not show my child (but is not depicting anything illegal). The parent mentioned that the child would not be allowed back to my community. My initial reaction was to delete the thread, but then I realized that I'd have to delete many threads that fall into the same category.

This specific subforum is only visible to members that have reached a certain post count. This means the user has to first register to the message board and second be an active member of the board. This is NOT an "anything goes" subforum, but does contain content for "consenting adults" - a phrase we utilize when they first enter the subforum. This subforum is not a large part of the community. I just looked and it contains less than 2% of the posts on the entire board.

This is my first complaint about the subforum. I do not want to raise the age limit of joining the community. This would be punishing the vast majority of the younger members for the actions of one parent. I'm also not sure that validating user age is appropriate. The seems to be a impossible goal. 

How do I protect younger members from more adult content?