Having a restricted area brings up all sorts of issues in regards to not only to age verification of members, but also with advertising such as the Child's Play banner up the top. Although not allowing search engines to "see" these pages often makes it ok, its not worth it simply to have an area for what exactly?
I understand your reasoning for wanting a restricted section, but apart from the showing of nudity (partial or full), what other topics would be discussed? Is it not possible that these could be discussed in public viewing? If nudity is about the only thing that it would be good for, then its probably not worth the effort to create a whole new forum when there is plenty of that on other websites.
I'd be more for a VIP forum, where select members (first chosen by staff, then by other VIPs) get a private area. This form has worked quite successfully at other websites, and serves as a reward to good members. Members also often benefit from larger upload limits, or flood control bypass, etc.