Hrmmm. Interesting quandary. I've never had problems with DH, and I used to run a fanfic archive on their servers years ago. Unless someone complains to them directly, you're probably in the clear. I've known of other fic archives on DH that have underage stuff. In fact, the only shut down of a fic site I've seen in my 10 years of fandom was due to outside sources, not DH or a hosting service itself. Honestly, I wouldn't worry about it.