Although I prefer to breed two litters at once to give me a foster mum on hand, I've now cut my litters down to just three a year. This is because I am terrible at letting the babies go, so I very easily end up with more rats than I can keep the way I want to. I feel the need to keep my rats in the same way I did before I was breeding, with daily freerange and plenty of attention. Once the number of cage groups reaches eight or more this becomes stressful.
I continue to prefer to let my does reach an older age before breeding. There are a lot of differing ideas about the ideal age to mate rats. It makes sense to me to wait until bucks are getting on for a year at the least, as any hormone related temperament issues have probably surfaced by this age. The best age for does is more open to debate. The myth that pelvic fusion occurs after a certain age has been debunked, different people give differing minimum weights for breeding does. I've read some interesting articles lately suggesting that by breeding does young we are selecting for early infertility, which is possibly linked to earlier aging.
Again, as I gain experience my preferred mating age has come down a little. This is because leaving it until later can mean that by the time you realise that your plan A isn't working, it's also too late for plan B. I'm now aiming at breeding my does from around 8 months old and my bucks from 12 months.