It has come to my attention that there is no catch-all tag for under-age material on e621. I had initially operated on the assumption that "cub" was this catch-all tag, because all underage anthropomorphs and feral-form creatures are tagged with it even when they are obviously not cubs (fish, birds, reptiles, to name a few). A mod was kind enough to point out the error of this assumption, when I was upset to find that an image of an underage human was not tagged with "cub." Apparently, the only solution at the moment is to also blacklist "loli" and "shota." This is unintuitive, and therefore a ticking time bomb for less-informed users living in areas of the globe where underage material is illegal.
My solution is simple: Add tag implications from "loli," "shota," and "cub" to "underage." At the moment, the "underage" tag contains only 16 posts, despite the site having 357 posts in the "cub" category, 315 posts in the "loli" category, and 304 posts in the "shota" category. Many posts in these categories overlap each other, which is why I incorrectly assumed "loli" and "shota" were already implicating "cub."
"Underage" is a much more intuitive tag for blacklisting purposes. Quite frankly, when people want to blacklist underage material, it makes the most sense if they can simply blacklist the tag "underage."
Updated by SnowWolf