Someone told me that originally mutton was the meat from 18-month old wethers and that there was a different term for old ewe meat. I think it was based on the French ? (Any of you ex-pats able to shed more light on this one?)
I have now eaten (all my own) hillbred and grassfed lambs, commercially reared commercial breed lambs, 2 yr old hill wether and old ewe (hill type.)
If I could only ever eat one type of sheep meat again, it would be 2-year old hill-reared Swaledale wether. Cooks like lamb, eats like lamb, lean, tender - and WHAT a flavour! Butcher describes the carcase as 'very poor' - meaning not enough meat on it for him to make sufficient profit (since it costs the same to butcher a 'poor' carcase as a good one - and more to butcher a fat one as they have to do loads of trimming etc.)
Next favourite, surprisingly, is Charollais x lamb taken off its mum at 4 months-ish. Pellets had been available and the lambs had been eating them but were still suckling. Butcher happy with conformation / amount of meat but found lambs to have insufficient fat cover. However the meat cooks incredibly succulent and absolutely delicious.
Old ewe is great but does need long slow cooking and a lot of skimming.
Texel type lambs fed plenty of cake, ready at about 4-5 months, not too much fat cover, are what the butcher wants to buy. The meat is very very tender but in my view there is less flavour than the others I have tried.
Supermarkets want lambs up to deadweight 21kg max (meaning around 40kg alive - not very big at all) and pay bonuses for good conformation and penalise you for fat. Some do ask for 'predominantly grass-fed' but mostly so far there is no recognition of flavour, tenderness, etc.
And finally, coming back to mutton (sorry for rambling - it is a subject close to my heart as you can tell!), everyone is always telling me that there is a big demand for mutton these days so I can sell it, can't I. Well, no, actually - marts are very happy to sell hoggs now and for a little while after the new season lamb becomes available but after that any older animal is lumped in with cast ewes and rams. As far as I am aware there are no buyers specialising in buying 18-24 month old wether meat for all these places that allegedly want mutton. (Apart from buyers for kebab & curry houses, I guess chinese restaurants, etc.)
If you want to sell mutton at the moment (up here at least) it's farmers' market, farm shop, mailorder, friends and relations. Auction marts, supermarkets and butchers are not interested.