@Kaarme
I'm making mostly guesses here, but he says in chapter 18 that he started living in a dormitory when he was a student which resulted in him not knowing his dad got remarried. Takaomi would've been born after the remarriage, so that's likely a 10 year difference minimum. "When I was a student" is pretty vague since it could mean anywhere from middle school to college. In the school festival arc Takizo (gaudy guy) mentioned that it was the last school festival for him and Koharu, so if he's the same year as Takaomi (I don't remember if he is or not) then they're 17-18.
If we're being generous and say Hirofumi started living in dorms when he was 12 (first year of middle school) then he'd be 29-30, though that's a really weirdly long amount of childhood spent never spending time with his dad (the fact that he remarried would kind of come up if they actually spent any extended time together in person).
That makes me think he started living in dorms as a high-schooler or even in college, since that's when kids typically start getting independent (wouldn't be weird for him to not spend any time with his dad). That'd make his minimum age 32-33 (high school starts at 15 in Japan).
It's not like I can confirm any of this tho, and I wouldn't be surprised if the author didn't actually put that much thought into it.
'Course, I completely forgot they're third years so it wouldn't actually be illegal. The fact Rino looks like a middle-schooler made it completely slip my mind.
Man, I really spent way too much time on this.