Ideally you want them to be at least 5-6 weeks old before you buy them, as they'll survive and grow much better without mother then (they are weaned enough to technically survive on their own at 3-4 weeks, but the longer they get mother's milk the better really).
As a general rule, anything that looks "cartoonish" and like a real baby is too young. Once they're old enough they look almost like miniature rats or mice, rather than squinty babies.