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 
"Non applicable"
"Are you sure about that?"
Tbh I support this measure because theres no reason for characters who look like mid twenties to be canonically children.
I gotta say, I'm not really a fan of the idea of deciding characters "look" like adults and therefore shouldn't be children in the story. Sure, these characters could be 25, or they could also be 15. 15 year olds can look pretty much like adults, and it isn't their appearance which determines whether they should be treated as such. It's their maturity and life experience, the invisible development of the brain, which matters.
The way I see it, there's really no such thing as a character "looking like" an adult. Did you not know any tall bearded kids in high school?
I had a huge bushy beard at 15 and a lot of people thought I was mid 20s
I was pretty baby-faced but I had the voice of a 25 year old when I was I was 14 lol
I know that because I'm almost 25 now and I have the same voice (and also people constantly said that online)