Most stop breeding at about 10 years. If you atop by 9 or 10 it prolongs the does life. However, I know someone who has Kikos and one of her does is 13 years old and still going strong. The doe still has her teeth, still runs with the herd and is completely able. A goat can live to 15-17 years. That is the regular life expectancy of a goat who was retired at 10 years old.
I think you really need to assess each individual goat. Some age more gracefully than others. Can your female keep weight on? Do they still produce plenty of milk? Can they maintain a decent weight when nursing kids? How are they getting around (obvious signs of arthritis)? Does she easily get sick or parasite problems? Does she birth unassisted or is she starting to need assistance? Are the kids coming out healthy or do they need a lot of help to stay alive?
oh wow didnt think they lived that long im glad i would hate to get goats and get attached and then they die with in a few years and dont have to trade out my does every few years cant wait until i get my first goat