With just educated guesses we can't be sure but I would say the number of fake minors on BL forums is even higher than 50% but most of those will post one or very few posts then disappear.

I do the same treat the person claiming they are a teen (I've never seen anyone claiming to be under 12 post on a BL forum) as if they are real but keep in mind they might be fake so do not get personal with them a all. I know of a few posters here who started off below or right around 18.