Rats can successfully mate at 5 weeks old ... Which is why its suggested to seperate them at 4 weeks.

If a rat hasnt been bred by 9 months stay away from it for a breeder. Around 9 months the pelvis fuses, if they havent had a litter this can lead to complications and even the death of the animal.

Normally its suggested to breed them at about 3-4 months. I personally go by weight ... once they hit 160 grams (large) they are good to go.

Bryan