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 
Its a fucking dumb trope and I'm tired of it. Watching kengan asura and the girl who keeps stalking tokito ohma for marriage is underaged. Keeps on saying and doing suggestive shit. Keeps talking about having his babies. There is no fucking reason she should be underaged none at all. Its not funny or cute or any of that shit. It doesn't add anything to the story.
On the bright side, they mostly stop doing things like that in Kengan Omega