I think She is 24-26, Lets Assuma She was 17(could be 18 too) When She graduated from High school, If I do not know this wrong, University is 4 years in Japan so She graduated while She is 21-22 I do not think You can just be ready to make a lesson as soon as You graduated and You should start as an assistant teacher so Hino happened after She graduated (If being an asisstant teacher means different Please inform me) That is 1 year (She is 22-23 now) She looked like She was that İce woman Even in The Main characters' first year cause None of them (Fumino and Rizu) including Nariyuki knew anything about her non ice persona and MORE than 2 years have passed since then Which would make her 24-26 (You can argue that Hino happened When they were first years and They simply did not know Mafuyu that much, Which could give a slight possibility of her somehow being 23 but I think It would be forcing it and Asumi Who is 1 year older than them also did not seem like She knew Mafuyu's early persona)