There really isn't a specific age or set guideline. I retire based on pregnancy history. If a female has a difficult pregnancy or delivery I will give her a rest them breed her again. If the same problems arise the next time I will usually retire her for her own safety. The same goes with a bad mother. If she doesn't care for her babies two litters in a row she is retired.
Slow breeders are different. Sometimes it just takes placing them with a different male to get things going. I was given a retired breeder from a friend because she has quit producing. I placed her with one of my males and she gave me a bunch of beautiful babies. I kept her in breeder until she turned 15. At that point she started having trouble loosing baby weight. She still loved being a mother and so I gave her a little kit just after it was weaned and she mothered it as her own.