The age a dog is ready to actually compete is dependent on so many variables, including breed, the type and amount of training, the conditioning of the dog, the dog's maturity (mental particularly). etc. I have had dogs not ready to mentally handle competing at 18 months, and those ready at a year. I assume NADAC used 18 months because that was what USDAA used, and NADAC grew out of USDAA to a large extent.
That said, AKC allows dogs to start competing at 15 months, and I have never seen any data that that has been a problem.
If people could be trusted to do what was in the best interests of their particular dog, I would say allow the competitors to decide when to begin showing their dogs. Since people are often overly eager to get into the ring, I would not advocate that. But i would like to see maybe a test period allowing dogs to enter at 15 months, maybe requiring that they jump a lower height. It would be interesting to see the results of such a test.