@Lili Said
A simple point though, if the 18+ forum was only visible to registered members over the age of 18, I'd assume that meant that anonymous people who had not yet registered wouldn't know it was there until after they had registered, so its presence wouldn't "attract" certain types of new registrations since they wouldn't know it was there until after they'd registered. In that same vein, a teen member wouldn't know to falsely register as 18+ in order to see the forum since they also wouldn't know it was there prior to registering. So not letting people change their age in their profile after they've registered would stop people from peeking by changing their age.
In addition to that, Google would never know you had a sex forum because Google would be indexing the forum as an anonymous user, and as such, would have no access to the 18+ forum, since it would only show up for logged in registered users over 18.
Lili there are plenty of forums on the net that do have sex forums.
If you are a member of those sites, you'll find that it's often a very different experience to TFS.
In my experience, it turned the forum into a kind of dating site. Rather than enjoying reading and making posts, I spent a lot of time blocking PM's from people who were only there to get their rocks off.
It was fun for the people who loved the sex forum. For the members over 18+ who weren't seeking that in the first place, it became a nuicanse. That's why I'm here instead of there.