there is no way to prove some anonymous user is underaged unless they explicitly admit it. They can claim being adult high schoolers (it's possible, I was one myself), or claim they are just flat chested while posting their bewbz and everyone would know better to not pry into it even if they feel like it's a lie
they just need to be smart enough to not admit anything, and not let their heyuri existence overlap with anything else so stalkerfags wouldn't have a field day