Apparently they can breed as early as 5 weeks of age, and the males mature a little slower than the females. So I would except around 2-3 months of age. You need to breed them before 4 months of age, or it may not be successful. With age, the number of babies, and frequency of matings will decrease, so you need to replace older mice with younger breeder mice.
Consistent handling, a quiet environment. Allow the males a day or two in the cage before introducing the female, they are territorial and need some adjustment time before mating. Rotate the females through the male's cage, but not the other way around. Females need 4-5 days at least in that cage for their estrus cycle. The female can get pregnant 24 hours after birth, so leave her in the cage with the male. Most males leave the babies alone. Give your mice nestlets-nesting materials and some environmental stimulation like a wheel to run it, etc... Also12 hours of light.
at 10:32 AM on Dec. 16, 2010