linlindesu, I've already reviewed the law that applies to this and I want to be honest, the law itself is a gray area.
Yes, a character typically has a "physical age" and a "mental age". In many cases the characters physical age and mental age don't coincide; however, what the law applies to is their physical looking age. Now, lets be realistic. The age range from about 15-20ish can be very misleading for a certain group of women (even a larger range for some, smaller for others). Some of them, physically, it is difficult to tell. Now, at that point when you take into account the fact that Anime, Manga, Hentai, and most anything else within the "Anime Community" is drawn, a lot of defining features are taken out to leave you with somewhat generically drawn characters that lack several of the items that you would typically use to tell their age.
Now, am I using this as a reason to say that anything that looks questionable is OK? Not really, what I am saying is that, that is the issue where determining age of a character comes into play. Yes, there is content explicitly drawn to look well underage, there is content within the realm of question, and there is content well over that age. When hentai/doujin doesn't have a defined age, what features does the law use to determine age?
You mention Bible Black explicitly listing on the cover that all characters are 18+. I looked at the cover, I don't see any such label. I do see on the back, and I quote "a junior college Magic Club," I also see the line "all characters depicted in sexual conduct or in the nude are aged 19 or older." Know what? This actually takes place in high school, how many high school students do you know are 19 or older? So, if I take a characters are 18+ sticker and slap it on a <insert_series_here> doujin, are we good? Further, actually Nanoha doujin and H-Content is allowed as in season 3 they are both 19... now bare in mind if they obviously draw them to be of a non 18 age... your risk.
You do also bring up a good point, mass media taking things out of context... though, you also have to remember, this happens often. How often does it happen where a reporter (maybe purposely) doesn't get the whole story, publishes something entirely out of context, and then needs to fix it... only to have that fix on a last page or somewhere unlikely to be noticed. Second, not the intention of my panel or what I want to do with it, and also a personal opinion of MINE, not the conventions, but maybe the media and the general public need to wake up and realize this is drawn... aka not real. I could go into pages on this; though, again, absolutely not the point I'm using this panel for. To be honest, a lot of what I'm going to go over can be part of any genre of anime... I'm just applying it towards mahou shoujo. You're welcome to think what you want about it, and make recommendations on it to help warn people from the legal gray line; however, this is all good clean... well.. echii fun >_>
I do mention cardcaptor sakura as it is a mahou series, but do you honestly think I'd be that dumb to include any actual adult content in regards to that series?!? You've got to be joking... While I'm sure adult content exists in Japan of this series, there would have to be some pretty good explanations of how some adult form was brought here... 10 years into the future... yeah... it better look older (note, I'm sure this exists somewhere). Also, Tsubasa was a step in that direction... In the art examples I use, at least those that are adult pieces, have features that indicate they are 18+.
I do want to make one other point; albeit a point of question and attack, what is the general setting of most animes... and even hentais? High School... Yes... Further, who has seen any of the AMV Hells?