I can understand why you'd assume I'm underage from that post but I am actually not underage.
Truth is 4chan was bound to be inhabited and used by underage users despite its rules because 4chan has the "anonimous hackers end edgelords" and whatnot, this attracts rebelious teenagers leading to an influx of underage people.
Like a porn site, there's little that can be done to figure out if a person is truly underage or not without them outright admitting since the underage are already (I assume) at least 20% of the general 4chan populace.
The underage have won.
The only thing you can do is cope