Is there a mom cat or are you fostering? If there's a mom cat, she'll get round to teaching them when necessary. My kittens were/are litter box trained by 5-6 weeks
Mine were litter box trained by about 3 weeks old. I started stimulating them into the litter box and using their paws to bury it. They caught on very quickly!
When I've had orphan kittens I usually stimulate them over the litterbox so as soon as they are able to control their own bladders/bowels they start using the box themselves... With my current kitten Church I caught him (feral) at about 6-7 weeks old. At first I kept him in a big cage when I wasn't right there watching him for safety, and he started using the litterbox pretty much immediately.
If they are still with mom, they will imitate her within a few weeks. I've had kittens show interest about 4 weeks old. But I keep the pan on a washable surface cause they are not always accurate, nor always use it at first.
Be sure to use a shallow pan (inch high) and clay or corn litter - not the scoopable kind. A tinfoil cake pan works good till they are big enough to use a small plastic pan.
After about 6-7 weeks old you can use other litter if you want.