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 
marie honoka and now (well, 5 years ago?) nico seem 14-16. idk what to tell you. maybe look at the face designs or story movies? science girl is a very tropey child genius, marie acts like a 12 year old, and honoka is explicitly a high school student
all that said, the devs don't really care about details, honoka is older than ayane by a few months and a discord person said shimbori didn't realize they had done that by having both be 18. anyway i'm doxed as a doa nerd now thanks OP.
This is subjective? Yeah I just dont perceive that shit. I wish it was more understood that subjectivity exists and people have different experiences with media. Like "Marie acts like a 12 year old" is just, not coding for me. I dont see it.
Then again I've not played the games lmao. I just see screenshots of their still designs and fan art. So maybe it comes across in cutscenes. And yeah I dont see it with Nico either since you mentioned her. Probably even less than the other two actually since she's dressed as a professional.
The only thing I can get is Ulysses point about them clearly being designed with a certain audience in mind. But thats not going to click for me ever because of the way my brain processes it. The face thing doesnt help because I know plenty of baby faced adults lmao. Some even post on this website and get annoyed when this discourse comes up for that reason!
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.