Lamb is meat from a young sheep - raised to be eaten young.

Mutton is meat from an older sheep, generally from sheep no longer good for wool production, too old to bear lambs, etc.

Mutton is a relative rarity outside the farm gate in shops and city butchers .. in an economic sense as soon as a sheep is big enough and well fed enough to be sold on to super market chains, why invest further time in that animal?

Unless, of course, wool production and| lamb production (ie. older ewes and some rams).

That said, lamb is quite rare in Japan, it's eaten primarily up north in Hokkaido.