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 
underage on paper characters who look and act older was so kids could relate to the characters, idk what's behind the loli thing, 4000 year old dragon is a pretty old meme, feedback loops in marketing somewhat explain the proliferation but not the root social causes.
That isn't always the case.
Fire Emblem had some very questionable "4000 year old dragon" type characters as breeding stock that had child-like dialogue, for example.
I wasn't going there to begin with. I was talking about the proliferation and normalization of those trends and how they interact with the consumers that continue to expect more of them while also getting effectively trained for that expectation. It doesn't even truly matter anymore how it may have started as much as it matters how it happened, continues to happen, and in the case of kiddie creeping gimmicks in entertainment, how the trends demand more of the same whenever possible to maximize profit from the audience primed for it.
yeah i meant like shonen anime main characters
As far as I know, shonen anime main characters aren't typically sexually objectified as a primary marketing gimmick.
EDIT: Seems that apparently they often are. I didn't know; haven't seen enough of them.
My friend, what are you talking about? Have you seen the main female characters in shonen anime? Almost all of them are significantly sexually objectified. My Hero, Fairy Tale, One Piece, you name it.
Apparently not nearly as many as I should have. Not really my jam overall, so I didn't know.