I've made a
characters/seiyuus page for this show, with pics of the characters and their seiyuus, and a bit of info about each seiyuu.
The cast is doing well, and there are some "names" among them: Hanazawa Kana, Kaji Yuuki, Hirata Hiroaki, Endou Aya (the narrator), Itou Miki (Saya's mom), Namikawa Daisuke. But there are a lot of unknowns in major roles, too.
Only a couple of seiyuus are playing their characters at both ages (said to be 12 and 14). Those are Kana-chan (Maria) and our apparent star, Taneda Risa (Saki). Shun at 12 is played by a woman, Toudou Mai, but at 14 by a young man who appears to be around 14 himself, Murase Ayumu.
I've included a couple of characters who haven't appeared yet, and could be considered
spoilers (although it's pretty clear characters like them are about to appear). Anyway, don't click through if you are extremely spoiler-phobic. Otherwise, it's no problem. I didn't feel they spoiled me at all, personally.
I got some of my info from Japanese Wikipedia, however, and had to look lightly to avoid seeing outlines of the full plot, lol.