I forgot to mention that i have seen japanese authors call this trope exactly this. For example, on page 40 of #179568|Momoiro Nurse|834 the title says 大人ロリっ女医 which literally translates to "adult loli doctor"
hmmm im not sure, we have [underage] already proposed, some variant of joshi kosei for elementary and middle school, and a few other proposals that could be searched alongside loli to get this i think
ill link what im talking about later
btw i know about "legal loli" term, but "legal" implies it should be "lawfully right" and that's a redundant way to describe it. And the character doesn't need to be 18+, as this trope also covers high-schoolers who look like elementary school girls. Basically any character who isn't a child but still looks like one.