See, I'm not convinced that image is a particularly good example, actually. To be rather blunt... how old would you say she'd look if you were to give her boobs? I'd say if she had boobs, she'd look at least 18, if not in her 20s -- especially given the youthful facial features and small statures commonly found in modern Japanese women.
So essentially, her design isn't particularly young-looking, it's just the flat chest that makes people assume she's young. But... erm... flat chests aren't exactly that uncommon in Japan either, to be blunt once more.
Maybe I've just been conditioned by years upon years of watching anime and playing Japanese games, but if I were asked to guess an age for that character, I absolutely would not go anywhere below 18. She doesn't look very young, and I really think it's the flat chest, and ONLY the flat chest, that makes people assume otherwise.
But ironically, depicting a character with a flat chest may actually be more realistic and less sexist than depicting a character with a huge chest in order to guarantee she looks to be "of age."
-Tom