I join with the others - 8-10 weeks.
I've been caring for a mommy feral for over a year, her kittens were first seen mid December 2008, their eyes were blue. One kitten (a female) hasn't gotten all that big -- even though she was nursing (along with the canned meat and premium dry food I was feeding them) until she was about 4 months (even after they were spayed).
She just doesn't seem to have the appetite that her brother has. She's really healthy - just not that big. Her mommy isn't that big either. The thing is,-- if I was just now seeing her -- for the first time of her life, -- I would guess that's she more like 6 months, when in reality - she's probably closer to 11 months. I'm guessing - since her eyes were blue when I first saw her last December, that she was probably 6 weeks, meaning that she would have been born sometime in November.
She's been fed well, is healthy - I managed to trap her in February 2009, and get her spayed (along with the others- yeah!!!), it's just that she's little, and as cute as she can be.