Female Russian hamsters can become fertile as young as 4-5 weeks of age, with males becoming sexually mature a little later. If males and females of similar ages are kept together in mixed sex pairs or groups they can therefore start to breed as young as one month old, but will more usually start to breed at around 3-4 months of age. If a young female is paired with an older male though she will often breed much younger.
Breeding a female whilst she is still growing herself is not recommended as it can detrimental to her health, affect her own development, cause the female to reject the litter, or result in a poorly developed litter. Therefore it is best to pair a females with a males of a similar age, rather than an older male, to avoid the female breeding too young.
Breeding Russian hamsters should not be undertaken lightly as they can produce a succession of litters as little as 3 weeks apart, although they may occasionally have a break in between litters and may not mate so often, if at all, over the winter.
Females often become sterile at around 12 months of age but males usually remain fertile for most of their life.