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 
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