Let's be real, just to get it out there, the "18+" rule is only a means for publications to absolve themselves of any legal liability by the model not being "of age" at the time of employment. It stands to reason, or at least this would be the converse logic, that a model's well-being is solely their own responsibility once they are legally deemed to be adults, and not that of the client employing them. It does not come from a place of safe-guarding working practices for models, but rather circling the wagon on themselves in advance of the next Boston Globe/N.Y. Times article.
Once you look at the "rules" from that vantage point, it becomes clear that "fairness" was never in the cards.