I found my six week-old kitten using the litter boxes used by the mature cats here. At what time should I consider getting rid of her little kitty-sized box and making the switch for good?
Well if she definitely won't eat it, then I don't see a problem switching over. Demi used clumping litter pretty young, I think he was a couple months old.
If you're sure she won't eat the clumping litter, than it's probably ok. Six weeks is still very young, though. Are the "big cats" litter boxes spread around the house. Don't want her to suddenly "have to go", and not be able to make it to a litter box in time. And do you have at least 1 litter box for each cat, so there's no chance she'd find all the boxes occupied.