I don't plan to tup mine as ewe lambs, but last year hadn't the option to keep the little girls separate. I managed them as in-lamb ewe lambs, of course. All but one lambed, every one of them did absolutely fine. All lambed unaided, outside, all cleaned their lambs, every lamb fed. All the mothers did well. There' were a few losses, but none at the point of birth or in the first 24 hours.
All the mothers grew well, and at the end of the summer I couldn't see a difernrce between the ones that lambed and the one that didn't.
These were pure Shetlands, Shetland crosses (Manx x and Castlemilk Moorit x) as well as 1/4 BFlLs.
So whilst I do prefer to give them the extra time to grow, my experience is that, managed correctly, Shetlands and other primitives can lamb successfully at one year old.