As L&M says, it's called hogget here and can be from gimmers, shearling wethers or shearling tups (away from the breeding season)
With some breeds of sheep selling meat as hogget is the norm - for example with Hebrideans and other primitive breeds. This is because they grow slowly so don't reach a useable weight in the first year, but by the time they reach a good slaughter weight the meat is close-grained and firm. Thus you can rear grass-fed animals which are slaughtered at 16 months or so. If you can do this without having to feed concentrates, just hay, then the extra expense on small numbers is warranted by the extra carcase weight.
However, I don't know that it would be worth it with a quicker finishing breed, which would perhaps just turn to fat. Someone who breeds quick maturing breeds can answer that part.
I know that with Hebridean hogget meat, and 3 year old Soay (they can take that long to reach a good weight) the meat is very different to young lamb - more flavoursome while maintaining the melt-in-the-mouth tenderness. How much of this is due to the breed and how much to the greater maturity of the animal and the slow-grown meat I'm not sure. We now only eat our own hogget.
Mutton is something else again and tends to be from older ewes which have been culled for being broken mouthed, having mastitis etc. We have eaten this too, although I tend to make it into burgers or sausages. I have also eaten wether mutton, mostly 7 yo, which was extremely fatty (some was from primitives and some from quicker finishing breeds, but all were fat, on grass only). They were my one-time fleece flock.