As for my phone, the tabs are on the top. You can choose "Unread" and apparently it's everything you have not read. On the bottom, I have "Latest," "Forum," Search," Messages," and "More." If you go to "Forum," it lists out all the forums. I clicked on Home Base and it says Topics (20 of 4,353). I think that should cover old threads.
If you have not noticed, you scroll all up, then there's this "Pull up to load more." It'll load more until I'm guessing you've reach, in my case for the Home Base forum, 4353 threads are displayed.