As a kid I assumed no one knew how old these two were because they were mysterious ninjas, but it turns out Tecmo just redacted their ages in the English translation for obvious reasons. It's funny they went with this route instead of just bumping both characters up to 18 years old. I guess Tomonobu Itagaki refused to tamper with the sanctity of his precious DOA canon 
spoiler
I'm sad to report the PS2 version of DOA2 seems to be one of those early PS2 games that's interlaced to hell and back and will look like a jagged mess no matter how much you upscale it in PCSX2 
Ashley in particular was an edge case where she wasn't labeled as a child but was given child-like dialogue lines and mannerisms (and to some extent, appearance), but it was the first one that came to mind where dialing back that particular setting of the dopamine treadmill in the remake was noticed, and caused outrage.
It's a numbers game for the corporations making these. The specific choices they make are often done with massive teams of marketing data researchers and the like, and what you may like in a character may not even be the intended appeal that they were going for when it came to mass appeal with an increasingly ravenous
demographic.
Yeah. Guess I'm losing the macro implications in the jungle of my micro experience with it.
Big corporations are all about the macro implications of every decision, even stuff that seems absurd like saving a few pennies by having a cheaper version of an essential part of a vehicle that can crash the vehicle if it fails... or something catastrophic like "no-knock" lead added to gasoline for generations or partially hydrogenated soybean oil added to food to somewhat extend shelf life at the cost of long-term human health.
Those are more extreme examples, but ones where it looks different when zoomed in, but when zoomed out enough, the pennies add up, especially if paying the fines or bribing the judges/congresspeople is cheaper than reverting the cheaper/more profitable choice.
There's a reason so many anime/manga/video game makers make demographic-targeted decisions to sexually objectify characters at as young an age as they can get away with: it's what they've already primed their primary intended consumers to expect, and that's reliable revenue.