I've been wondering about this too and I'm really glad I saw your post! Sajou definitely makes a comment in OTB that he has two years left and "marry me" at 20, which has always made me think he was 18...or at the very least NOT twenty yet. I agree it seems like they're about 20 now. But you're right about Sano, because he talks about having already waited five years to sleep with Hana in OTB, which would put Sajou and Kusakabe into their 20s, I think? I used to think the two stories were set at different points in time, but they interact in the high school reunion chapter, so...
TL;DR I'm also confused about the ages.
I don't know why but this has been bothering me. What are the ages of the characters as of now? I know that Sajou and Kusakabe are likely 20. But Hana-sen and Sano's ages confuse me. As well as Hibiki and Satoshi sensei. In the Occupation to Beloved it's implied that Sano is 20 because he can finally have sex with Hana-sen which makes no sense as he's suppose to be at least 3 years younger than Sajou and Kusakabe. The same goes for Hibiki technically because he was still a highschooler when Sano was a high schooler but is now at pastry school. I know this is random but it's been bothering me because I can't figure out the time line of the stories. Like are all the stories simultaneous events happening at the same time or are they their own stories centered around each pair happening in their own specific time and just relayed to us in sequential order? I mean Sano is clearly an adult who's living with Hana-sen by OTB and Kusakabe went to visit Hana-sen when he was 18 because Sajou mentions there's still two years before they can get married so I feel like that proves that events are happening simultaneously. So are their ages screwed up just to speed things up in the story? Does someone have an explanation for this?