I've heard from quite a few people in various working-dog sports that a sire loses his potency of passing on traits after around 7-8 years of age. I am not sure if this is purely anecdotal or based in biology and genetics.
I personally see some merit in this, just basic genetics, and would not choose to bred to an older stud (older than 8) or get a pup from one.
Bitches can be safely bred to even age 10, if they are healthy and have had litters before. In some ways, it is healthier for an intact bitch under around age 9 to have a litter than to not.