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 
In Japan, once you enter work life your life is basically over so they romanticise the fuck out of your teenage years and your early twenties
Any character over 30 is seen as a decrepit old fart. In MGS1 Solid Snake is forced out of retirement for one last job and several characters tell him he's aged and looks like shit. He's 33 but they act like he's pushing 50.
tbf doesn't Snake age faster because he's a clone or something?
True, but I'm not sure if Kojima had thought of that plot point at the time he was making MGS1. It's definitely there in MGS2.
The way Snake is treated is consistent with other Japanese media though