Originally Posted by AncientDNA

It can happen if the female produces the pheromones that stimulate the male's mating response. It would really depend on if the female is biologically ready to mate.
I once had an 18 month old borrego 'accidentally' become gravid. She was already pretty large and ended up having three live young & two DOA's.
Still, I would suggest keeping them separately, but if not, just keep an eye out for amorous behavior from the male. One mating does not guarantee a pregnancy...and I would suggest against it in an 18 month old.