(or under 16's like the UK, or 15's like Sweden, or 14's like Germany, or 13's like Spain. Because different countries have different laws. Surprise).
You misunderstand the difference between legal age of consent and child pornography laws (drawn stuff often falls into this category, insane as it is), those two are different in all countries you've mentioned and are usually 18 (as suggested by Interpol I believe).
They were careful to prevent it!
Do characters actually die or become increasingly less desirable by majority of clients as they are aging? I don't recall even seeing that code in WM so age attribute is used very scarcely. The bottomline is that they'd be better of without it all together...
As for "Plausible deniability", Vanilla version of WM will land you in jail in US if I understood the precedents correctly. Crazies mod is less likely to do so as it is released without images and when paired with characters like Rangikou, Youruichi, Erza and etc. preferably all set to above 18 is absolutely safe...
===============================================
If we're talking law, in precedents, words like "original source" had been thrown around. So the perfect example would be Post-Skip version of Hinata from Naruto, a very popular character who's age in the source is 16 while noone in their right mind could tell that from 99% of her pics (that could not be judged underage by parameters set in those laws based on appearance).
So if she's specified 16 in the game and you'd find yourself in UK/Australia/US etc. (in unlikely event that your PC/Laptop finds it's way to the law and they decide to waste their time prosecuting you instead of some actual criminal) that prosecuted people for Drawn stuff before and she's also 16 in the source, chances are that you screwed. If she's specified as 18, it is a hell load less likely to fly and 17 would prolly be somewhere in the grey area...
===============================================
Another bottomline is that age can be hardly used in the game logically since it's about a magical universe where a girl can be immortal/don't change with age while bring mortal, drink some potion to stay alive/young/combination of both so actually using that in the game will bring more issues that it would solve... (pics don't really age either btw...) making it a semi-useless field that brings a shitloads of trouble and repeated discussions without realistic added benefit...
Edit:
Edit: ah drugas beat me to the deniability argument...
And you beat me to a couple of mine
