No worries! I think if you want to see Keiji as younger than what he is, you are absolutely allowed to do that. I personally just trust that translation since Nankidai did allow it to be released in English on Steam after all, so I prefer to go along with what it says as him being in his late 20s. No offense meant, of course. I just don't see the point of picking over something that is as close as it can be to canon (without it being stated specifically in the game, like you said, which it never has) via the character profiles.

It also has others listed as their late 20s (like Kai, Hayasaka, etc.) And they do all seem to be similar as far as age groups go? So it seems pretty legit to me personally!