You can litter box train a cat at any age. While kittens can start around three weeks old, if you’re fostering or adopting a cat that’s not trained, you can start as soon as you take it home. The reason you want to wait until three weeks to start training kittens is that they’re starting to develop their motor skills and observational abilities. It makes it easier for them to learn from their mother and siblings if they have them.
Many kittens adopted out from shelters are at least 8 weeks of age. If you adopt a new kitten, it likely already has litter box experience and will simply need to be reminded where its new litter box is located.