we have eaten 7yo Shetland mutton and it was delicious - grass fed only on our farm and as fat as butter! Makes brilliant curries, slow roasts (with mezze kind of spice rub on) etc.
^this
Primitive sheep mutton is fantastic meat, no question.
I have also eaten mutton from several other breeds and types and it's all been fabulous. When it's that old, it needs to be hung for a minimum of a week after slaughter, and will want long slow cooking, but oh my goodness, you may never want to go back to lamb.
Mule mutton mince makes lasagne so good you never want to make lasagne with any other meat ever again. You have to pour a pile of orange fat off your plate before you eat, but it's still the best lasagne you've ever eaten! (These were older Swaley Mules, 5 years old and older.)
Mutton sausages beat most pork sausages into a cocked hat, doesn't matter how old the sheep was. They don't need a load of fancy ingredients added to make them taste good, in my view, but lots of people like to spice 'em up.
Old moorland Swaledale ewe mutton, joint or chops, roast slowly with veg and red wine (so basically a pot roast I suppose) - flavour so good that lamb will forever after taste washed out.
(Basically, as moorland farmers we liked to sell our lambs, and drafted ewes who were good enough to do a few more years on an easier farm, so we would eat the old biddies who didn't deserve to have to go through a slaughter mart. We ate like kings
)