Since when was Amu 17?

Are we sure that's age? Because several of these characters either aren't 17 or have no official age.

The Kana reads "17 Saigoro" but I'm not sure what that means, since I can't understand the points 1 and 2.

And why is Miku the only one that's naked -_-;;

Some of them really are 17, right? (Manabi, Ranka, Otoha, Konata, and Kyou.. Not sure about Kotonoha.)

But what's with the long-haired Yuki?

Lol, Manabi is 17? 17-7 I think...

"17 Saigoro (17 歳頃)" means "*about* 17" so they aren't necessarily 17.
Some of them might be 16, or 18.

I don't recognize a fair number of these...does anyone know which respective show each girl is from?