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 
it's a lot in the cutscenes and voice direction, yes.
I feel like one of the only "she looks/acts underage regardless of canon" character in fiction where i've seen it is Fire Emblem Awakening Nowi who 100% looks and acts 12 to me (and should not have been dressed like that or made marriageable lmao). But even that was largely more because of the appearance than behavior. I swear the "acts like a kid" thing just goes over my head with like genki girl types. Rikku (who is canonically underage, but regardless) is another one like that where people will cite her behavior and I'll just be like "what? she's just upbeat?" And yes I was aghast and confused when I found out Rikku's canon age and still percieve her as college student aged.
Idk I genuinely think my brain is broken these conversations make me feel like an unhinged freak because everyones seeing things I dont see. Like i've had private conversations with people who agree with me but public ones always seem to go the other direction lmao. Like I guess I'll fall back on "subjectivity exists" but it is weird to me and kinda alienating.
As one of those aforementioned baby-faced adults yeah I'm with you. I've never played DoA so I assumed it's something about those two characters you can't pick up on with just their appearance. Though the sexualization of an 18 year old high school student is still gross to me.