The age you keep lambing a ewe will depend in part on her breed and breeding, your system ('commercial' or just a few sheep), the quality of pasture, elevation, hardness of winters, feeding regime etc. This spring we had twins from our 15 yo Hebridean with no problems but she had a bit of extra feeding throughout the winter and we kept a very close eye on her condition throughout. Commercial ewes are I think culled so young because that is when they tend to start losing their front teeth and it is not cost-effective to carry a large number of ewes through that stage. We breed for long-lasting teeth and might cull a ewe young if she starts to lose teeth before about 8 or 9, as the tendency can be inherited. What we do find though is that while a ewe is in the process of losing her front teeth, she will struggle to get enough nutrition to carry lambs, but once all the front teeth have gone she will be able to eat properly again with a little support from extra hard feeding, and be back to lambing again. Our 15 yo lost her teeth at 13 and fortunately was carrying only a single that year. We would expect about 50% of Hebrideans to go on lambing until they are about 12, but Soays don't seem to be so long-lived, lambing up to about 10. Shetlands we would expect to lamb up to about 10 too, although a few will go on far longer than that - these figures are for our land, our management and will be different for others.
From the genetic point of view, if you are breeding breeding stock, especially primitives which are expected to be long-lived, it is better to breed from older ewes and tups, so you can know their longevity, how long they keep their teeth, what their feet are like after a good few years and so on.