They can live a long time, and it varies a lot. Average is about 12 years, but they can live as long as 20 or as short as 8 years.
IMO, age doesn’t play as much of a factor as condition and health. You can breed them as long as they look nice and healthy and haven’t had age-related birthing complications. I’ve seen does break down at 8 years but some still breeding at 12.
We have a pretty young herd, since we don’t have room to retire more than a couple does. Our oldest turns 10 in November, and we only have a few 7-8 year olds. We currently don’t supplement them any different.