I think it depends on the reason you want to show your dog.
If you are looking for socialization and an introduction to the show ring, then the younger the better. Puppy matches are a great place for both you and a puppy to practice for the "big time". Just don't go into it expecting to finish from the puppy class. Can it happen? Absolutely. Just look at Pepe. But it usually takes longer that way.
If you are looking to win points toward a championship and wish to present to both judges and the fancy your dog in the best possible light, then I say wait until they mature. The majority of Newfs (not all lines or individual Newfs)get better with age and will typically finish faster when they are a finished product.
I usually will enter my puppies in a few shows while they are 6 to 9 months old. We do not expect to win but rather it is a time to get them used to the grooming table, show sights and sounds and a strange person looking at their mouth! I then will usually put the puppy away for a while, letting them grow into themselves. During this time I work on obedience and the other working disciplines.
This being said, I now am showing a 15 month old girl who is doing very well in the ring. She is a large girl who matured at an even rate and very quickly. In short, there is no rule only what is best for you and your Newf!
