Dimensions was banned in some countries for violating child pornography laws, as the stated ages for some of the characters was below 18. Marie is aged 18 (and the other characters' ages were bumped up too) in order to prevent this situation from happening again. This however is completely divorced from the designs of the characters themselves. Other DoA games were released in these countries with no issues, simply because the same character's ages were listed as ??, rather than stamping them with a fictional ages, that would cause them to fall foul or real life laws.
There's a big difference between a situation like that, and what is going on with Marie Rose (and this is also why there's such a different reaction to it). The way a character is designed should logically supersede whatever fictional age is then attached to that design. Nobody is attracted to an "age", they're attracted to physical features, characteristics and mannerisms. If you create a ninja girl, with an adults body proportions, adult mannerisms and behaviours, and adult clothing sets, you're not going to have that character appeal to those attracted to children, even if you then say she's two years old (cyborg characters are typically a good example of this). However, if you create a character that matches a pubescent teen in every single one of these categories, then you will appeal to this audience even if you say the character is 45 (although 18 is obviously preferable to make this adjustment slightly more plausible).
The whole "I know someone with a petite figure like Marie" comes up all the time. This may be true, but her frame is a single element of the overall equation. I sincerely doubt you know someone with Marie's frame, a similarly childish face, that would wear a child's strawberry one-piece swimsuit (with the frills and everything) to the beach, twirls in circles to express happiness, does the whole cat-like crying thing to express disappointment, etc... basically, if you were to drop her off amongst a crowd of actual children, she'd blend in, and nobody would be able to discern which one was actually not 12. None of you have an adult friend that's like Marie Rose.. I'm calling BS on anyone claiming they do.