My sister could sign up, and I could appear on the videos. This is going to be hard to prevent, especially in cases of 17year olds like in the story where it's not immediately clear from the material itself they're under 18.

I can't really think of any system that would prevent this while also not being highly invasive.

It's only substantive example is a single individual who was able to fool a moderator that family members passport was theirs, and that pales in comparison to... Well literally anything you might compare to.