I'm 23. Keisuke's age was never explicitly given, although implied to be in his late teens/early 20s twenty years after 20XX.
I've been using what canon gives me to extrapolate a birthdate, based on his personality, hints in the text, and astrology. (Yeah, I know, but it has good character building use.) From what I can tell, the earliest he would have been born was 2005, and the latest would be 2095. Based on how similar his canon is to our present day (just grittier), I'd say it's probably on the lower end. I need to do more research but I'm guessing if we were in the same world, he would have been born a few years ago or a few in the future, at most ten.
Tl;dr Depending on who you ask and what criteria you are basing age off of: nearly nine (source material's age), not born yet, 18-23, or 27-32. I personally see him as being 20 or 21 in the source material, and that's the age I picture him at. We haven't been together long enough for this to impact anything, but I think I will likely imagine him growing older as I do. I do sometimes think about the fact that he's likely not born yet, though, and it makes me feel a little weird.
Yes, there's a fair amount of us who do that. Not everybody does but there's nothing wrong with it.