we have had a couple of heb ewe lambs caught by accident yes they do lamb and rear a lamb but the sheep never catches up with it's peers and ends up being a runt in the flock as it's providing for a lamb rather than growing it's self. In the commercial world they reckon you should really only breed from ewe lambs over 40kg - albeit a soay will never reach that - Given a choice i wouldn't
The 40kg is not an absolute figure, but a percentage. The Eblex guidance (
linky) states that ewe lambs should be 60% of their mature weight at tupping. So a commercial ewe whose adult weight is 70kgs should be 42kgs at tupping.
If an adult Soay is 25kgs, then a ewe lamb should be 15kgs at tupping.
Some of the other tips in the Eblex document are to feed pregnant ewe lambs 20% more concentrate than you would pregnant ewes, through gestation and lactation. And to creep feed the lambs, and wean the lambs no later than 14 weeks. It also suggests taking any second lambs off, so that ewe lambs are only rearing single lambs - and to feed more concentrate if they are rearing twins, and potentially, if the lambs are eating enough creep, wean as early as 8 or 9 weeks.
They did not find that breeding ewe lambs adversely affected lifetime performance, indeed the opposite. The studies did not cover primitive breeds, however.