Okay, so I was looking at Keiji’s page and I saw that he referred to Nao, Kanna, and Sara with the honorific -chan in the Japanese version of the game, but refers to Reko without the honorific.
So the honorific -chan is used to refer to refer to younger people usually, or to refer to yourself. So Keiji only uses it for girls who are younger than him.
Reko is 23, and since he just calls Reko by her name without any honorifics, I think Keiji would have to be younger than 23, but older than 20. So between 20-23 is his age.
Someone might’ve already said the exact same thing as me, which is okay. I’m glad I noticed that, though. And maybe there’s another reason why Keiji doesn’t use honorifics for Reko, or maybe the Japanese translation that we have in the wiki is incorrect, so maybe I’m wrong. I’d like to think I’m right, though, you know?