mutton is the meat of domestic sheep. It has a stronger flavour and deeper colour than lamb. Formerly it was much liked and eaten in Britain; Mrs Beeton (1861) commented that it was ‘undoubtedly the meat most generally used in families’. It was taken from animals between three and five years old, among which wethers (castrated males) were considered to yield the best meat.
Looking at the question from a longer perspective, as a general rule (before the industrial and urbanized age) domesticated animals were kept for their secondary products rather than their meat. The great exception to this was the pig. But sheep, goats, cattle, and horses, to name the four most important, were useful for their wool, milk, transport, and traction rather than their meat. The young of these species would not, therefore be killed for the sake of a meal. The rule was not, of course, universal. Some of the males might be surplus and some food requirements might be better fulfilled by younger, tenderer meat. With sheep, the general rule was that the adults were eaten. Lamb was a comparative rarity, and seasonal at that. And many found the flavour of lamb displeasing: to their eyes, strong. The situation only changed with the vast ovine populations of Australia and New Zealand, the larger body-weights of improved breeds and the development of refrigerated transport. Now, in Britain and much of Europe, mutton is the rarity.