Baby rats are seperated by sex at 5 or 6 weeks of age for a reason LOL. The question isn't whether they can breed but whether should they breed. Even for feeder stock (rather than fancy rat pets) you want a successful breeding with lots of big healthy young and a healthy strong mom so I wouldn't recommend allowing a female rat much under 4 months of age to breed (some fancy rat breeders will tell you 6 to 9 months of age for first breeding). I'm not sure what age yours are.