8 weeks old ...it depends on the brand of the shampoo so be sure to read the label. If needed though, you can always give the kitten a bath with plain old dawn (NOTHING ELSE) it's mild enough so it shouldn't hurt the kittens skin-but only do this no often than once a week
You can use dawn dish washing liquid instead and johnson's tear free shampoo! Do a good lather and let it set for about 5 mins! BTW don't put the soap directly on the cat or dog! Lather in your hands 1st b/c you could rinse the poor animal for weeks and never get the mess out! Lamo
Baby shampoo instead
Around 3 and 4 months and if your kittens have a lot of fleas you should ask a vet.
8 weeks no less,fleas can be a killers for young babies
Around 9 weeks