Generally a kittens eyes change at 7-8 weeks old. It may vary with litters however. I've had litters of newborns who've opened their eyes at 4 days old (they aren't supposed to open till near 10), and I'm sure it's not uncommon for kittens eyes to change at a younger age. Teeth can gauge how old a kitten/cat is as well.
http://www.cathelp-online.com/health2kittens3.html (google kitten teeth, and there's plenty of info).