Every doe is different. I have retired does at 7 and I have a 10 year old that just aborted :/
Anyways since every doe is different I go off each individual. If they seem to have had a hard time kidding, like it just took way more energy then it should have, then they are retired. My does are notorious for giving it all to their kids and loosing some condition while nursing but they bounce back fast after pulling kids. If they don’t bounce back fast, they are retired, or if they loose way more condition then I would like. Or like the 10 year old I’m taking the aborting as a sign so she is done.