She can be bred this fall. Lots of people (myself included) like to wait until they are about a year and a half before breeding as they are more mature both physically and mentally and there is a smaller chance of complications.
There are lots of people that routinely breed ewe lambs and have no issues, so it is really up to you.
It's generally said that a ewe will come into heat when she weighs approx. 80% of what will be her final adult weight. Nature will therefore decide when she is ready to breed.
I place my lambs with the ram in the autumn,but only the bigger and heavier ewes become pregnant and seem to do very well.
I like my ewes to be over a year old before they lamb. But I've had some big fast growers that were a bit younger when they lambed. No problems but they hang with older experienced mammas and I think that helps allot!