Same here - if a ewe is sound in all the right places and produces fine lambs then she stays, whatever her age. We have bred twins from a 15 yo - (Heb of course which is a long-lived breed).
I would not put them through a mart. Call me soft - I feel my old ladies have served me well and will have as easy an end as I can manage for them. I don't send one on her own, nor do I feed them to the dogs. Biddies make wonderful burgers and sausages - I get the butcher to mince the lot then I try a variety of different flavours. They are particularly delicious with a curry sauce
I have though managed to find two 'retirement homes' for groups of older ewes, at about 8 yo. Both new owners wanted them as lawn mowers and to look good, and at that age they would get 4 or 5 years from them at least.