Yeah I checked out porn when I was underage. May or may not have checked certain bdsm themed websites as well (Not this forum)
The problem is that unless someone basically says they're underage, there is no way of knowing. And of course, if they do mention it, then the obvious step is to inform moderators to block said person. But not to sound like a pessimist, despite tools like IP blocking, there is really very little someone like this website can do to prevent underage users. If they live in my neck of the woods, they can easily get their ip changed, steal someone elses, use the university/school wifi. Unfortunately, it's up to the parents/guardians to monitor these minors.
The best we can do in this forum is be helpful to everyone and respect the local law and the rules of this forum.