I prefer not to think of Misuzu that way either, but keep in mind that she's described as flighty and immature, and that she doesn't really act her age.
I haven't seen her age definitively stated anywhere, but personally, I'd put her at sixteen -- consider the shot of her birthday cake in episode two:
I count twelve candles, and it looks like 1/4th of the cake has been removed so far, implying that it would have had sixteen candles on it originally.
Minagi and Kano should be pretty much the same age -- Misuzu introduces Minagi to Yukito as a classmate, which I interpret as being in the same year in school. Kano might be a little younger since she refers to having seen Misuzu's face a few times, but doesn't know her name, which could mean they're in different years, or just in different classes. Anyway, that's just my best guess...if there's a more concrete statement of the characters' ages that I've missed somewhere, I'd be glad to have it pointed out.