I've got nothing against the underage warning as such, 16 or 18 doesn't matter at all. It's the wording I've got an issue with. Just because the law defines someone as a "child" people in general don't see teenagers as children and to define them as such in a fandom challenge is absurd. (Not to mention having some very unfortunate implications.)

What's wrong with the usual formulation, used for example by AO3: [Underage refers to descriptions or depictions of] sexual activity by characters under the age of eighteen