Rats need to be with their mothers until they are 4.5 weeks old. Generally, it's a good idea for them to be separated but stay wherever they are until 6 weeks, just to make sure they are eating, drinking okay and are well. You will not be able to buy a newborn rat, unless you're looking to feed. You wouldn't WANT a newborn rat, because you wouldn't be able to keep it alive, unless you are willing to feed it formula every 2 hours, night and day, stimulate it to go to the bathroom, and keep it warm. Even then, the chance of survival is about nilch. Since Petsmarts don't sell feeders, you won't find a newborn rat at Petsmart... That they'll sell you, anyway. At least, they shouldn't.and why can' t you buy a rat until 6 weeks?