I notice folks who put ewe lambs to the tup never say how many crops of lambs they expect to get from them over a lifetime. Our go to the tup at 18 months old and we expect at least six crops and average 8, sometimes running to 11. Most will have twins every time. The older the ewe the more confident you can be of her health (otherwise you'd have culled her or she'd have died), which she will hopefully pass on to her offspring, and the less likely she is to be slow in mothering up, in my experience.
I'd expect them to have a single as a lamb and twins every year after until there teeth go or they develop some other cullable offence! Without being funny, to average 8 crops of lambs across a whole flock, it must either be a fairly small flock, or you are very lucky and have to cull almost no ewes, or have no ewes die before their time! That wasn't meant offensively by the way, i'm just impressed.
With regard to ewes lasting, I cannot imagine why lambing as a well grown ewe lamb, would lessen its productivity or life span? I have a load of hoggs that lambed this year, that have been weaned now. They have all done their lambs well, and they are as well grown as those that didn't lamb, and the older ewes. My biggest concern to be honest is the fatness of the few ewe lambs who didn't get in lamb and so have run on a year of just eating. To be honest, they should probably have been sold, but I need the numbers and have not put a foot wrong in any other way, so they are still here.
As i've said, I will be tupping about 65 of my own ewe lambs this year, that's a lot of mouths and a lot of grass if they don't produce a lamb until they are basically two years old! I would hope to get around 60 lambs out of them next spring, which even at £50 a lamb, will give me £3000 to add to my bottom line, which is not to be sniffed at. Especially when the alternative is renting ground to just graze sheep!
With regard to the unknown quantity of the ewe lamb, all first time lambers here, go to a terminal sire (a little french pig), which allows me to assess their worth, and not have to add their lambs to the flock.
With regard to age. . . . . the oldest ewes being tupped this year will have been born in 2008. They have had a single lamb as a ewe lamb, and a set of twins every year there after. . . . . . without being ever fed anything at all, what so ever!