Our Southdowns often produce lambs up to 13 years old, probably helped by the fact that they have excellent teeth all their lives unlike, say, the Beulah. They're generally retired when milk production can't keep up with demand. I do like a shed of mostly old girls, who know what they're doing and just get on with it. Had one case of twin lamb disease about ten years ago - probably due to stress caused by an abscess on her leg hidden by the wool, but she recovered. Not yet had a case of grass staggers. Horses for courses, I guess.