In addition to the kanji 「歳」 preceded by a number, there are some other clues that a pretty young model is actually a dangerously-young model. In a video store, for instance, the “U-15” section will be chock full of 15-and-under girls, and as the shelves fill up, you may even find a separate (squick!) U-12 section (or their equivalents “JC” and “JS”). U-17 seems quite innocuous by comparison, especially when attached to the formidable figure of Natsuki Sawada.
And in a rare case of a number making it easier to ogle, there’s Nana Hiratsuka’s 30+α Bluray release.