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.