My boys seem to have mostly gotten the idea of litter training (or at least pooping in the general area of the litter boxes, if nothing else). However, my girls just never seem to catch on. Instead, they sleep in the litter boxes, dig out the litter, or ignore them. I want to say they are approximately five to six months old. When rats get past a certain age, does it get difficult or impossible to litter train them? Or can it be done at any age? Should I just keep trying with the girls and hope they get the idea eventually?