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 
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):