Storyseeker thats an old myth that is untrue, but older does are not recommended to breed UNLESS you know the lines well (we do not mean the parents...that is not a known background). We are talking about knowing back a minimum of 3-5 generations, knowing the health, illnesses, temperament (and changes of) causes of death of EACH of the rats from EACH litter.
forget their age, they could be genetic timebombs waiting to happen.
I shall give a few examples.
1) Sweetie came to me, had her 2nd litter a day after arriving.
All 6 girls stayed and they are prone to horrible URI's and now tumors at 18 months of age. One young girl had a mammary tumor removed at TEN months of age
Mostly it was 3 girls who had to be constantly on medicatins to keep their chronic URI's in check, and now I have all 6 girls sick. Mom Sweetie just died at 21 months of age after wasting away like she was ancient. :/
2) a rescue friend took in a younger mom rat who had five 3-week babies with her (was told she usually had litters of 4-5). All went well until Beta gave birth again (post-partum pregnancy). She gave birth to 3 pups (1 died soon after) and then her labour stopped. She was rushed to the vet and had to have a pitocin injection to deliver the other 12 babies.
Then she refused to nurse them. My friend started hand-feeding ever 2 hours. The babies dropped like flies until there was only 4 left. They seem fine so far and just made it to 3 weeks of age. My friend on the other hand is exhausted.
THEN the three 6 week old boys of the former litter had diarrhea, and severe lethargy for 1 day. They recovered with fluids, etc. Three days later they all died suddenly. NOW one of the 2 remaining older girls is barely alive...THIS is what you could be producing.
Btw just because the mom is lovely tempered doesn';t guarantee the babies will be.