When considering breeding, wait till your dog is AT LEAST 15months old for small breeds and 20months for large breeds.
You want your dog to be %100 physically and emotionally mature (you will be able to tell when they are calmer and a little less playful IF you know your dog) before breeding.
If your dog is 18months old and isn't emotionally mature, then do not breed him.
However, just because your dog seems mature at 1year, that DOES NOT mean you still breed him or her.