Mice are generally full grown around 6 mnths, but rats can take as long as 9 months to become fully grown, however, I have had rats that grew for just under a year before they stopped.

Mice and rats can breed as early as 6-8 weeks old. This the reason we separate them soon after they are weaned since weaning takes place around 3-5 weeks, depending on the Mom. I have noticed, a female kept with a male, who is impregnanted right after giving birth, will wean thier young right around 3 weeks, so that she is not nursing the older litter when the new babies come. Keep in mind, pregancy for them is right around 21-27 days, but I have seen some drop right at 19 days. The ones that drop right around 19 days or so, are generally those that have had many many litters. Also- to be kept in mind, just because they are sexually mature at 6-8 weeks, does not mean it is the right time to breed the females, the ones that have babies at too young of an age usually end up being bad Moms for one reason or another. The best time to breed a female is right around 4-6 months. After 9 months, if a female has not had any litters, you must becareful because thier hip bones have "closed" a bit and her birth canl is not going to allow passage of big babies to easily, taking the chance that one may get stuck, losing the mother and any babies still inside. It seems that if they are bred during the optimal time of 4-6 months, that they dont end up with problems like that, not sure why, thier hip bones still "close" a bit. This has just been my experience with rodent breeding. Gonna have to look through all my records, somewhere I have an article on rodent breeding ins and outs.