some topics in my womens only section of another (younger-style) Christian forum include: masturbation, brazillian waxing, menstrual periods, labour and birth stories / questions, and a number of other things, you can see why these things would be more openly and honestly discussed if it were in a fairly closed - gender invironment?? (I would personally appreciate discussions about certain things with older women and not with a whole lot of teens which is the main population of said other forum)
You might also find that said other forum has some nice little privacy stuff on their girls only section, which you might be well served to read. They ask that a topic discussed doesn't go outside of the girls only section to promote trust. Things like the discussion of masturbation, periods, brazillians, pregnancy stuff are pretty likely to come under the heading of topics that members of that other forum might not want mentioned all over the interwebs.
Young people are dealing with a lot more than we know (or would like to admit) and i think that the experience a lot of us older Christians could offer would be very valuable, excluding 'underage' people would be counter productive to this (as they could falsely declare their age anyhow).
You may also find that the advice of Older Married Christian Women Who Sound Like They've Got it All Together isn't always intrinsically correct - just because someone is a younger teen, that doesn't mean that they're devoid of a valid opinion.
Gabriel has brought up an exce;;ent point about not only having gender forums to keep (some degree of) privacy from the opposite sex but also from the general www world. I know it is possible to make a particular group / forum non Google or Yahoo searchable for example, and this may allow people to be more comfortable discussing certain things if their posts weren't going to be able to be found by just anyone (say, by a user name search)
A girls only section relies on trust - trusting that the others who are granted access to it won't abuse the trust placed in them.
Although you haven't mentioned any members of the other forum by name or the specific content of any post, I'd suggest that you be careful of how much you share of things that you've been trusted not to in future.