Yes, complicated. That is why even laws use loose language like "appears to be" and "underdeveloped" versus chronological age (e.g. when to card a consumer of alcohol or tobacco). Many games and other media, and most businesses that rely on sexual, youthful appearances play with that ambiguity between appearance and chronological age (e.g. all the "teen" porn).
Rule 7 determinations on this site rely on gross appearance of underdeveloped characters. It is nearly impossible to tell the age of a character in a two-dimensional story. Instead, they make a guess based on the extreme appearance of child-like versus adult-like. Subjective but "most" people would on the decisions made here.
At the risk of reigniting an ignorant argument on this thread, the dev uses this discrepancy to her advantage (and there is nothing wrong with that). Chronologically and mathematically, it is possible to determine the rough age of the characters. However, their appearance remains the same. So, perhaps they are under 18, but their appearance is the same as when they are over the age of 18. Just a thought.