i would say about 4 weeks, try some wet food, or dry moistened with KMR. what i did with icis' litter was get 2nd stage KMR and make it thin like water, and warm. once they ate that, i kept making it thicker, till it was gel like. then i went to mixing the 2nd stage with dry food into a mush, they LOVED it! once they ar hooked on that, its easier to get them to eat wet and dry food.
Most kittens are not really interested in solid food till 5-6 weeks old. You can start with a soupy mix of canned kitten food, KMR, or warm water. Offer it to them several times a day.
You may have to put some on your fingers and kinda "force feed" them so they get the taste. Feed the kittens on a shallow plate/paper plate - they will get messy so feed on a cleanable surface (tile, etc.) and you may have to wash them off after if they get too messy.
Every kitten is different... I have 6 kittens who are 5 weeks old and 4 kittens who will be 4 weeks old on Wednesday/Thursday. Three of the older ones are eating canned food, and two of the younger ones are as well. I just keep putting the food down and every couple of days another one will catch on and start eating.
Lemme say sorry first because this will sound like an advert.
I started putting out RC babycat at 3 weeks (it's great for mom cats too) and have yet to have any problems. The rule of thumb is wetted kibble or wet kitten food at four weeks but since I switched to BabyCat I've had no problems at all! I will admit that the babies did and do get a can of wet every night though!