commercial finnished lamb is normally about 6 -7 months old and weighs 40Kg ish.
A lot of it will be a
lot younger than that - 14-18 weeks. Many hill-bred lambs will take longer to fatten, and even on good ground not all lambs will finish in that time, but a commercial producer with Texel sheep or similar will be looking to have the majority away before they're 5 months old.
There's a whole next tranche of course, of lambs which take up to 12 months to get to the magic 21kg deadweight - breeds such as Cheviot, for instance. Often they are bred on the hills, and sold at 4 or 5 months old as store lambs, fattened on dairy land when the cattle are housed over winter and sold after Christmas, leaving the grass cleaned and freshened ready for the cattle again.
The 21kg deadweight is a supermarket requirement - they found that if they buy lambs heavier than that, they get a lot of fat and not much more meat. They would, they tell us, be happy to be buying bigger carcases if only they could reliably get
lean ones.
Our local butcher likes his lambs quite a bit bigger - up to 26 kgs deadweight. But still lean.
My feeling is that there is an optimum age and size for a given breeding on a given ground. On hill ground, or old-fashioned unimproved grassland, I like to get the fullness of flavour of two summers, which requires a less commercial type than a Texel - ie, hill sheep or primitives.