If its a hefty well grown ewe lamb being put to a small ram, then maybe ......otherwise I would wait. Altho the experience they get lambing is useful, the likelihood of complications is greater, and it could check the ewe lambs own growth.
I usually try to be more conservative than any commercial farmer as a rule of thumb, and since even some of them dont risk it, I reckon its better not.
But interested in what others think, I guess it partly depends on whether theres a big commercial imperative to do so.