Drahken, the law usually goes with how old they actually are, not how old they look.
TV Tropes has or had a trope called something like "She's Really 5000" which addressed the Japanese manga and anime stories that have a young-looking character in cheesecake or hentai situations excusing it as claiming she's really older.
Panther, I didn't read the link because I read the original stories in Batman Family & Teen Titans and the age situation was a problem then, just not addressed in the stories.
The logical solution would have been she was the Earth-Two Two-Face's daughter, but a Teen Titans story showed that she was the daughter of the Earth-One Two-Face as verified by Two-Face.
Years later in The New Teen Titans, Marv Wolfman made the age difference canon, but didn't explain what a real answer is or why Robin was so stupid as to not put 2 & 2 together before this.
When she started off it seemed like the creators wanted Robin (who had his own series) to have his own Joker, then apparently decided she might make an interesting new hero. Oddly enough she did use Joker's Daughter as a hero identity (dumb), Harlequin was an attempt to allow her to keep using the comedy crime fighting gimmicks, but with the cancelation of Teen Titans there was no place for her to appear and be used as a character falling into comic book limbo until New Teen Titans #50 brought her back addressed the age problem and later on she became the really screwed up character she is now.