Hill and upland farms may sell 1-, 2-, 3- and 4- crop ewes (so up to 6 years old) as a product of the farm. After that, they take more looking after on the hill, but could go on for another 3-5 crops on easier ground. In the north of England these are called draft ewes. We bought an occasional batch if we were short of replacements, or to bring in new proven blood.
Ex-BH didn’t like to sell his ewes as draft, never being comfortable that they would settle in a new farm after their very extensive life on the uplands along Hadrian’s Wall. So he’d keep them as long as their teeth and udders were sound, up to a max of usually 6 or occasionally 7 or 8 crops. At that sort of age (8-10) they’d be culled anyway, as his long experience had taught him that they would start to have problems if he kept them on beyond that. Plus, as a commercial farmer with commercial meat types, they’d still fetch a reasonable price if culled while still fit. I think a hill type bred pure can probably keep working quite a bit longer - I knew of a 17-year old Herdwick had just had her 14th lamb on the hill, one every year since she was 3 - but producing two Texel thugs each year on the hill is a big ask and the ewe needs to be fit to carry, produce and rear them.