When we started with sheep, Zwartbles, our vet recommended we bred the ewe lambs, as long as they have grown on well they are mature enough to do so, and if they are not ready they tend not to take.
If you do breed from your ewe lambs ideally they should be kept as a seperate group to older ewes in the flock, they cannot keep at the trough with older ewes and will not be getting the quantity of feed you think they are getting if they are in with older ewes and you risk problems such as twin lamb etc if you do not keep the ewe lambs seperate/ensure they have adequate food intake in the run up to lambing. You also need to ensure that you don't overfeed them and they cannot have the same intake as an older ewe, this is something to do with the fact that they themselves are still growing as well as needing to divert nutrients to the growing embryos and they can get metabolic problems.
We have had very few lambing problems with ewe lambs, in general they lamb extremely easily and more quickly than older ewes. Ours have been good mums although can be too overprotective and keep the lambs under their nose rather than let them drink until they learn about lambs needing the milk bar.
We lamb ours after the main group, aiming for early April so that the ewes have good grass available whilst they are lactating. You do need to bear in mind that a ewe hogg/shearling will only produce 60% volume of milk as an older ewe would too, so certainly if she has twins they may need more supplementary feeding than lambs born to older ewes.
I think if you are prepared to take the additional care with them and their lambs, then as long as they are well enough grown and an appropriate breed there is not any reason not to breed from them. Our vet said that they would then be in better breeding condition for future years. However if we wanted to grow a lamb on for showing as a shearling we certainly wouldn't breed from her in her lamb year.