Ok, so Kotaro was 32 when he died. I think we're getting closer to an estimate on how old Toshinori is now don't we?
Well, nothing new really? Toshinori gotta be several year elder than Kotaro. Nana left Kotaro when he was, like, seven or so? Just a kid. Seems like when she «adopted» (not really, of course) Toshinori, he was a middle school student (wearing gakuran). Seems like it was AFTER she’s left Kotaro, but we don’t know how much time has passed. And when she died - Toshinori was eighteen. Kotaro died fifteen years ago, so even if Toshinori is just seven years elder...
All in all, Toshinori gotta be in his late fifties.