There's not really an age limit but we've managed to toilet train ours within a couple weeks at the most. Unless you want to clean up their poop forever, I'd teach them as soon as possible while they're young and learning.
Usually as soon as possible when they are young. You CAN train potty train cats when they are adults, its just 20 billion times harder.
it doesnt matter the age but some cats do take more time to learn then others so start early.