I don't agree with idea. The forum could be used wrongly and start a whole new gateway into drama that is unneeded.
And if your only to talk about mental illnesses and other stuff then I find no point to it as people of any age can speak generally about it and speak about it anywhere off the forum (ontop of that, people can lie about things such as mental illness and that just goes into a whole other problem). Normally when I think about 18+ forums, I normally think of topics that are against FH's guidelines. (Swearing, implied sexual tendencies, and other no-no's) and that typically shouldn't be appropriate for this game's demographic.
Also, if we do have an 18+ forum this can increase in undesirable activities (drama, bullying, ect.) and also can put younger participants who would lie about their age at risk. I think something such as this doesn't belong on Feral Heart, and it certainly should be taken somewhere that it's okay.