It's honestly been so long since we made this game so I don't remember everything perfectly, and wouldn't necessarily have written their characters the same way if we did this now, but from what I remember - their age gap isn't much of a noticeable thing in the plot. It's only two or three years, which isn't exactly unusual in real life (think Ren might even have been 17 turning 18, at least I think we wrote him as a senior in high school?).
Sorry if that makes you uncomfortable! But it's not like a "fetish" thing in the plot, if that's what worries you. Hopefully you can still enjoy/did enjoy Kazuya's route if you decided to play.