I place food in at 3 weeks old and pull weanlings at about 25-28 days old.
By then they are mauling their mom for milk, and they are ready to be on their own.
If your feeding the rats you don't need to separate them at all.
I put all of my breeding rats together at weanling size and raise them up in groups, the males tend to learn their manners better when raised with females.
I've never had a female get pregnant before she was ready to be pregnant, typically around 14-15 weeks old and then they are dropping babies at around 17-18 weeks old.