[Warning: Slight spoilers for volume 5 of the ZnT manga ahead.]
Well, Hakaze's age hasn't been confirmed for good so far. There's only been a few hints:
- She's supposed to be older than Yoshino and Mahiro, and the age difference apparently isn't only a year or two (in volume 5 of the manga, she chides herself for "behaving like a teenage girl with a crush").
- She's also younger than Junichiro (the two of them refer to each other as "little sister"/"older brother" ("Jun-nii") respectively; also, there's an instant where Junichiro drops a few lines about Hakaze as a child). Now Junichiro's actual age hasn't been confirmed either, but he is a college student, a.k.a. in all probability somewhere in his twenties.
Personally, I'd think Hakaze would be about 19-20, but as I said before, it's just an estimate, and her actual age still has to be confirmed.
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When Hakaze was found dead she's still schooling right? looking from the picture of hers in school uniform when Yoshino and Mahiro visited Junichiro.Besides, she travels across time to the few years in the future from the past. So, technically her age is few years younger from the current year in ZnT. Er.. thats just my assumption though...