There is no correct age at which to breed a rat, and there are many equally valid preferences.
It was a commonly accepted truth that, at around six months, does would have their hips fuse and be more likely to die in labour. While this has some truth in guinea pigs, it’s absolutely false in rats, and so does not form a risk factor. Even so, breeding rats at 4-5 months old has advantages – the does are almost without exception fit, in their prime, and raise kittens beautifully. They recover very quickly from raising a litter. It allows you to get through generations at speed, which can be incredibly useful for test-matings, for realising the potential of outcross programs, or to reach a goal. Even breeder who often wait til later occasionally breed a younger promising doe simply out of the excitement of seeing what she will produce! It can become overwhelming though, with two and nearly three generations of rats in a year – while the rats recover quickly, it can take more of a toll on the breeder!
Most breeders breed does around 6-8 months. At this age they’re very definitely adults, they’re mature and almost invariably fertile. Often they’ll have had a chance to go to a few shows and prove their worth, but they’re still young enough that litters are an easy prospect and the does tend to do well. It’s a bit more of a relaxed pace for the breeder too, and a bit of a compromise position between the two extremes.
Some breeders choose to breed later than this, with 9-14 months being an oft-cited window. This extreme gap between generations allows breeders to gather more long-term data on a line before the next generation arrives, so it can be useful if you have concerns about a line having late-arriving issues that have proven problematic (lifespan problems, tumours, or similar), later breeding can provide a way of more easily monitoring those. As the rats get older there is more chance of infertility, and although most does cope fine with a litter, it probably isn’t ideal for the individual rat (like a 20 year old woman will normally find recovery from and child rearing easier than a 45 year old woman, even if they both are fine!).
If a line is used to being bred very early, then you may find that if you wish to breed later on, they are not fertile, as this is not something that has been selected for.
If the fertility is normally good at six months (does fall pregnant easily, give birth to good sized litters), I’d probably breed one at six months as normal, one at eight months.
If the second doe takes, favour her offspring in the next generation, and breed from those at 6/7 months and 8/9 months, and repeat, gradually pushing the age of breeding until they’re regularly fertile later on.If the second doe doesn’t take, breed from her offspring at 6 and 6/7 months … and repeat as above but maybe with smaller gaps between the early and late doe to ensure that you have more chance of fertility.
My personal preference is to breed does between 6-10 months, ideally at around 8 months. I feel this is a good compromise position between being able to get a good overview of the line and the pressures I am putting on the individual doe.
With regards to increasing fertility, I would also select towards kittens that are slower to mature – smaller, and look “kittenish” longer, and those with a racier and longer body type over the wider and shorter rats (thoroughbreds rather than draught horses) as my gut feel and experience says that they tend to become fertile later and stay fertile longer.