I don't think author's showing us their ages just because. I'm predicting an extra flashback chapter for the parents where the mom was a little girl and the dad was in his mid teens, and she confessed to him and asked him to marry her. And so he faithfully waited for her until she was an adult before dating an marrying her. She would have had Rintarou when she was around 23, so she probably did college too before having a baby.