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 
Is this about Marie Rose? She's the only character I can think of that could be read like this? Idk if thats the intent or not but Marie Rose just looks like a petite adult woman to me, 18-20. Should could pass for younger but I just... dont process things that way lmao? I've never been able to with Japanese media characters people say "look underage" unless they literally look pre or very early pubescent. Just not something that processes in my brain.
Like you mention Talim as well, with Talim I was surprised when I learned her age lmao. I again processed her as a petite adult (though I was myself younger than 15 when I played SC2 lol). Amy I suppose looks her canon age though even though I hadn't thought of it before.
Same with Street Fighter Lily as well. I''ve seen so much "she looks underage' discourse and I'm just like what? She just looks petite. Adults can be petite? And in her case there's no canon information otherwise.
Idk maybe I just have something broken in my brain because everyone says this lmao.
This isnt even a "please let me goon to these characters in peace" thing lmao I dont care about that. I just find the common perception around them odd because thats just not how my brain processes them.
Yeah, it's always seemed weird to me how characters that are canonically like 18-20 often get drawn in a way that scans as "30 or so" for me, and then characters that look roughly college aged are canonically 14-16. It feels like the animation equivalent of like live action high school dramas where all the actors are at least in their mid twenties.
There does seem to be something in Shonen stuff in particular, where the characters are basically adults in appearance and activity, but because they feel the need to make them relatable to kids somehow, they just say they're under 18, even though nothing suggests they are. Maybe they shoehorn them being at school a few times, but they don't do any school stuff ever like go to class or do homework.
I feel like there's a million different properties that have some kind of military academy setting so they can have a bunch of child soldiers running around
I'm glad I'm not alone on this lmao.
I was just thinking about this some more and how characters are being drawn in what I'm currently reading (ZOM 100), when I reached a great character recap spread at the start of volume 10. For the fun of it, I want everyone to look at this version with the ages and bios censored and try to put an age to each character, weighing both anime tropes and the context of this conversation and how the characters scan in a vacuum:
Hint:
The three guys on the right are all the same age, and the girl on the left is the oldest character out of all of them.
The uncensored image (mild story spoilers):
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.