Actually, if you want healthier, larger litters, get a female that is 5-6 months old. If you breed the females too young, they will likely have many problems, such as babies getting stuck in birth canal(pelvis not fully formed and is small), hemorrhage, small litters with small, thin babies, as well as the mother may not lactate or even have a clue what to do and will reject the litter. Breeding them at an older age usually ensures that you will have bigger, stronger litters, and the females will actually have a clue on how to raise the babies. It just makes sense, hm?